chore: update models in samples/ folder to be gemini 2.0+

Gemini 1.5 is obsolete and not accessible to new projects.

PiperOrigin-RevId: 795330977

contributing/samples/hello_world_ma/agent.py

@@ -131,7 +131,7 @@ prime_agent = Agent(
 
 
 root_agent = Agent(
-    model="gemini-1.5-flash",
+    model="gemini-2.5-flash",
     name="root_agent",
     instruction="""
       You are a helpful assistant that can roll dice and check if numbers are prime.

contributing/samples/human_in_loop/agent.py

@@ -39,7 +39,7 @@ def ask_for_approval(
 
 
 root_agent = Agent(
-    model='gemini-1.5-flash',
+    model='gemini-2.5-flash',
     name='reimbursement_agent',
     instruction="""
       You are an agent whose job is to handle the reimbursement process for

contributing/samples/output_schema_with_tools/agent.py

@@ -82,7 +82,7 @@ def get_current_year() -> str:
 # Create the agent with both output_schema and tools
 root_agent = LlmAgent(
     name="person_info_agent",
-    model="gemini-1.5-pro",
+    model="gemini-2.5-pro",
     instruction="""
 You are a helpful assistant that gathers information about famous people.
 

contributing/samples/workflow_agent_seq/agent.py

@@ -23,7 +23,7 @@ from google.adk.agents.sequential_agent import SequentialAgent
 # Takes the initial specification (from user query) and writes code.
 code_writer_agent = LlmAgent(
     name="CodeWriterAgent",
-    model="gemini-1.5-flash",
+    model="gemini-2.5-flash",
     # Change 3: Improved instruction
     instruction="""You are a Python Code Generator.
 Based *only* on the user's request, write Python code that fulfills the requirement.
@@ -38,7 +38,7 @@ Do not add any other text before or after the code block.
 # Takes the code generated by the previous agent (read from state) and provides feedback.
 code_reviewer_agent = LlmAgent(
     name="CodeReviewerAgent",
-    model="gemini-2.0-flash",
+    model="gemini-2.5-flash",
     # Change 3: Improved instruction, correctly using state key injection
     instruction="""You are an expert Python Code Reviewer.
     Your task is to provide constructive feedback on the provided code.
@@ -69,7 +69,7 @@ Output *only* the review comments or the "No major issues" statement.
 # Takes the original code and the review comments (read from state) and refactors the code.
 code_refactorer_agent = LlmAgent(
     name="CodeRefactorerAgent",
-    model="gemini-2.0-flash",
+    model="gemini-2.5-flash",
     # Change 3: Improved instruction, correctly using state key injection
     instruction="""You are a Python Code Refactoring AI.
 Your goal is to improve the given Python code based on the provided review comments.