chore: Adds a new sample agent that demonstrates how to integrate PostgreSQL databases using the Model Context Protocol (MCP)

## What's Added

- **PostgreSQL MCP Agent** ([mcp_postgres_agent/agent.py](cci:7://file:///Users/admin/git%20repos/adk-python/contributing/samples/mcp_postgres_agent/agent.py:0:0-0:0)): A fully functional agent that connects to PostgreSQL databases via the `postgres-mcp` MCP server
- **Comprehensive README** ([mcp_postgres_agent/README.md](cci:7://file:///Users/admin/git%20repos/adk-python/contributing/samples/mcp_postgres_agent/README.md:0:0-0:0)): Documentation with setup instructions, configuration details, and example queries
- **Environment Configuration**: Support for secure credential management via `.env` files

## Key Features

- **MCP Integration**: Demonstrates proper use of `MCPToolset` with `StdioConnectionParams`
- **Zero Installation**: Uses `uvx` to run the MCP server without manual installation
- **Secure Credentials**: Database connection strings passed via environment variables
- **Production-Ready**: Uses Gemini 2.0 Flash with unrestricted access mode for full database operations

## Technical Details

- **Model**: Gemini 2.0 Flash
- **MCP Server**: `postgres-mcp` (via `uvx`)
- **Connection**: StdioConnectionParams with 60-second timeout
- **Environment Variable**: Maps `POSTGRES_CONNECTION_STRING` to `DATABASE_URI`

## Testing

The agent has been tested with:
- PostgreSQL database connections (local and remote)
- Schema inspection queries
- Data querying operations
- Table listing and management

## Example Queries

Users can interact with the agent using natural language queries like:
- "What tables are in the database?"
- "Show me the schema for the users table"
- "Query the first 10 rows from the products table"

This sample serves as a reference implementation for developers looking to integrate PostgreSQL databases with ADK agents using MCP.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3182 from Vrajesh-Babu:postgres-mcp f3b3846abae37ae376d3554624ac2b1be82f7adc
PiperOrigin-RevId: 822865931

contributing/samples/mcp_postgres_agent/README.md

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+# PostgreSQL MCP Agent
+
+This agent uses the PostgreSQL MCP server to interact with PostgreSQL databases. It demonstrates how to:
+- Connect to a PostgreSQL database using MCP (Model Context Protocol)
+- Use `uvx` to run the MCP server without manual installation
+- Pass database credentials securely via environment variables
+
+## Prerequisites
+
+* **PostgreSQL Database**: You need access to a PostgreSQL database with a connection string
+* **uvx**: The agent uses `uvx` (part of the `uv` package manager) to run the MCP server
+
+## Setup Instructions
+
+### 1. Configure Database Connection
+
+Create a `.env` file in the `mcp_postgres_agent` directory:
+
+```bash
+POSTGRES_CONNECTION_STRING=postgresql://user:password@host:port/database
+```
+
+Example connection string format:
+```
+postgresql://username:password@localhost:5432/mydb
+postgresql://postgres.xyz:password@aws-region.pooler.supabase.com:5432/postgres
+```
+
+### 2. Run the Agent
+
+Start the ADK Web UI from the samples directory:
+
+```bash
+adk web
+```
+
+The agent will automatically:
+- Load the connection string from the `.env` file
+- Use `uvx` to run the `postgres-mcp` server with unrestricted access mode
+- Connect to your PostgreSQL database
+
+### 3. Example Queries
+
+Once the agent is running, try these queries:
+
+* "What tables are in the database?"
+* "Show me the schema for the users table"
+* "Query the first 10 rows from the products table"
+* "What indexes exist on the orders table?"
+* "Create a new table called test_table with columns id and name"
+
+## Configuration Details
+
+The agent uses:
+- **Model**: Gemini 2.0 Flash
+- **MCP Server**: `postgres-mcp` (via `uvx`)
+- **Access Mode**: Unrestricted (allows read/write operations). **Warning**: Using unrestricted mode in a production environment can pose significant security risks. It is recommended to use a more restrictive access mode or configure database user permissions appropriately for production use.
+- **Connection**: StdioConnectionParams with 60-second timeout
+- **Environment Variable**: `DATABASE_URI` (mapped from `POSTGRES_CONNECTION_STRING`)
+
+## Troubleshooting
+
+- Ensure your `POSTGRES_CONNECTION_STRING` is correctly formatted
+- Verify database credentials and network access
+- Check that `uv` is installed (`pip install uv` or `brew install uv`)

contributing/samples/mcp_postgres_agent/__init__.py

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from . import agent

contributing/samples/mcp_postgres_agent/agent.py

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+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+
+from dotenv import load_dotenv
+from google.adk.agents.llm_agent import LlmAgent
+from google.adk.tools.mcp_tool import StdioConnectionParams
+from google.adk.tools.mcp_tool.mcp_toolset import MCPToolset
+from google.genai.types import GenerateContentConfig
+from mcp import StdioServerParameters
+
+load_dotenv()
+
+POSTGRES_CONNECTION_STRING = os.getenv("POSTGRES_CONNECTION_STRING")
+if not POSTGRES_CONNECTION_STRING:
+  raise ValueError(
+      "POSTGRES_CONNECTION_STRING environment variable not set. "
+      "Please create a .env file with this variable."
+  )
+
+root_agent = LlmAgent(
+    model="gemini-2.0-flash",
+    name="postgres_agent",
+    instruction=(
+        "You are a PostgreSQL database assistant. "
+        "Use the provided tools to query, manage, and interact with "
+        "the PostgreSQL database. Ask clarifying questions when unsure."
+    ),
+    tools=[
+        MCPToolset(
+            connection_params=StdioConnectionParams(
+                server_params=StdioServerParameters(
+                    command="uvx",
+                    args=["postgres-mcp", "--access-mode=unrestricted"],
+                    env={"DATABASE_URI": POSTGRES_CONNECTION_STRING},
+                ),
+                timeout=60,
+            ),
+        )
+    ],
+    generate_content_config=GenerateContentConfig(
+        temperature=0.2,
+        top_p=0.95,
+    ),
+)