chore: migrate from gemini-1.* and gemini-2.0* to gemini-2.5-*

`gemini-1.*` and `gemini-2.0*` models are respectively deprecated and scheduled for shutdown on June 1, 2026. `gemini-2.5*` models are their successors.
No regressions in unit tests:
```
========================================================================================== 5583 passed, 2237 warnings in 84.91s (0:01:24) ===========================================================================================
```

PiperOrigin-RevId: 907663315

Author: google-genai-bot

contributing/samples/a2a_auth/agent.py

@@ -29,7 +29,7 @@
 
 youtube_search_agent = Agent(
     name="youtube_search_agent",
-    model="gemini-2.0-flash",  # Replace with the actual model name
+    model="gemini-2.5-flash",  # Replace with the actual model name
     instruction="""
     Ask customer to provide singer name, and the number of videos to search.
     """,
@@ -47,7 +47,7 @@
 )
 
 root_agent = Agent(
-    model="gemini-2.0-flash",
+    model="gemini-2.5-flash",
     name="root_agent",
     instruction="""
       You are a helpful assistant that can help search youtube videos, look up BigQuery datasets and tables.

contributing/samples/a2a_auth/remote_a2a/bigquery_agent/agent.py

@@ -39,7 +39,7 @@
 )
 
 root_agent = Agent(
-    model="gemini-2.0-flash",
+    model="gemini-2.5-flash",
     name="bigquery_agent",
     instruction="""
       You are a helpful Google BigQuery agent that help to manage users' data on Google BigQuery.

contributing/samples/a2a_basic/agent.py

@@ -94,7 +94,7 @@ def roll_die(sides: int) -> int:
 
 
 root_agent = Agent(
-    model="gemini-2.0-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/a2a_basic/remote_a2a/check_prime_agent/agent.py

@@ -48,7 +48,7 @@ async def check_prime(nums: list[int]) -> str:
 
 
 root_agent = Agent(
-    model='gemini-2.0-flash',
+    model='gemini-2.5-flash',
     name='check_prime_agent',
     description='check prime agent that can check whether numbers are prime.',
     instruction="""

contributing/samples/a2a_human_in_loop/agent.py

@@ -36,7 +36,7 @@ def reimburse(purpose: str, amount: float) -> str:
 
 
 root_agent = Agent(
-    model='gemini-2.0-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/a2a_human_in_loop/remote_a2a/human_in_loop/agent.py

@@ -39,7 +39,7 @@ def ask_for_approval(
 
 
 root_agent = Agent(
-    model='gemini-2.0-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/a2a_root/remote_a2a/hello_world/agent.py

@@ -66,7 +66,7 @@ async def check_prime(nums: list[int]) -> str:
 
 
 root_agent = Agent(
-    model='gemini-2.0-flash',
+    model='gemini-2.5-flash',
     name='hello_world_agent',
     description=(
         'hello world agent that can roll a dice of 8 sides and check prime'

contributing/samples/adk_pr_agent/agent.py

@@ -143,7 +143,7 @@ def get_github_pr_info_http(pr_number: int) -> str | None:
 """
 
 root_agent = Agent(
-    model="gemini-2.0-flash",
+    model="gemini-2.5-flash",
     name="github_pr_agent",
     description="Generate pull request descriptions for ADK.",
     instruction=system_prompt,

contributing/samples/agent_engine_code_execution/agent.py

@@ -66,7 +66,7 @@ def base_system_instruction():
 
 
 root_agent = Agent(
-    model="gemini-2.0-flash-001",
+    model="gemini-2.5-flash",
     name="agent_engine_code_execution_agent",
     instruction=base_system_instruction() + """
 

contributing/samples/application_integration_agent/agent.py

@@ -37,7 +37,7 @@
 )
 
 root_agent = LlmAgent(
-    model="gemini-2.0-flash",
+    model="gemini-2.5-flash",
     name="Issue_Management_Agent",
     instruction="""
     You are an agent that helps manage issues in a Jira instance.