Feat/expose mcps streamable http custom httpx factory parameter (#2997)

* feat: Add support for custom HTTPX client factory in StreamableHTTPConnectionParams

* Update src/google/adk/tools/mcp_tool/mcp_session_manager.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

* unit tested mock

* provide default - httpx client factory can't be none

* feat: Enhance StreamableHTTPConnectionParams with httpx_client_factory attribute

* fmt

* fmt

* refactor: Rename test_init_with_streamable_http_none_httpx_factory to test_init_with_streamable_http_default_httpx_factory for clarity

* isort

* fmt

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Kathy Wu <108756731+wukath@users.noreply.github.com>

src/google/adk/tools/mcp_tool/mcp_session_manager.py

@@ -25,17 +25,22 @@ import sys
 from typing import Any
 from typing import Dict
 from typing import Optional
+from typing import Protocol
+from typing import runtime_checkable
 from typing import TextIO
 from typing import Union
 
 import anyio
 from pydantic import BaseModel
+from pydantic import ConfigDict
 
 try:
   from mcp import ClientSession
   from mcp import StdioServerParameters
   from mcp.client.sse import sse_client
   from mcp.client.stdio import stdio_client
+  from mcp.client.streamable_http import create_mcp_http_client
+  from mcp.client.streamable_http import McpHttpClientFactory
   from mcp.client.streamable_http import streamablehttp_client
 except ImportError as e:
 
@@ -84,6 +89,11 @@ class SseConnectionParams(BaseModel):
   sse_read_timeout: float = 60 * 5.0
 
 
+@runtime_checkable
+class CheckableMcpHttpClientFactory(McpHttpClientFactory, Protocol):
+  pass
+
+
 class StreamableHTTPConnectionParams(BaseModel):
   """Parameters for the MCP Streamable HTTP connection.
 
@@ -99,13 +109,18 @@ class StreamableHTTPConnectionParams(BaseModel):
         Streamable HTTP server.
       terminate_on_close: Whether to terminate the MCP Streamable HTTP server
         when the connection is closed.
+      httpx_client_factory: Factory function to create a custom HTTPX client. If
+        not provided, a default factory will be used.
   """
 
+  model_config = ConfigDict(arbitrary_types_allowed=True)
+
   url: str
   headers: dict[str, Any] | None = None
   timeout: float = 5.0
   sse_read_timeout: float = 60 * 5.0
   terminate_on_close: bool = True
+  httpx_client_factory: CheckableMcpHttpClientFactory = create_mcp_http_client
 
 
 def retry_on_closed_resource(func):
@@ -286,6 +301,7 @@ class MCPSessionManager:
               seconds=self._connection_params.sse_read_timeout
           ),
           terminate_on_close=self._connection_params.terminate_on_close,
+          httpx_client_factory=self._connection_params.httpx_client_factory,
       )
     else:
       raise ValueError(