Add GeminiAgent for Google Gemini model support

[gavksingh]

Summary

• Adds GeminiAgent and GeminiAgentOptions for integrating Google Gemini models via the google-genai SDK
• Follows the same pattern as OpenAIAgent — supports streaming/non-streaming, system prompts, retriever integration, and pre-configured client injection
• Adds gemini optional dependency group (pip install agent-squad[gemini])
• Gracefully degrades when google-genai is not installed (conditional import)
• Added comprehensive unit tests and a Vertex AI integration test

Files Changed

• python/src/agent_squad/agents/gemini_agent.py — new agent implementation
• python/src/agent_squad/agents/__init__.py — conditional import for GeminiAgent
• python/setup.cfg — gemini extras group with google-genai>=1.0.0
• python/src/agent_squad/types/types.py — GEMINI_MODEL_ID_GEMINI_2_0_FLASH constant
• python/src/agent_squad/types/__init__.py — export the new constant
• python/src/tests/agents/test_gemini_agent.py — new test file
• python/src/tests/pytest.ini — registered integration marker

Tests added

New file: python/src/tests/agents/test_gemini_agent.py (8 unit tests + 1 integration test)

Unit tests:

• test_gemini_agent_requires_api_key_or_client — ValueError when neither provided
• test_gemini_agent_client_injection — pre-configured client bypasses api_key requirement
• test_custom_system_prompt_with_variable — template {{variable}} resolved correctly
• test_process_request_success — non-streaming returns ConversationMessage
• test_process_request_streaming — streaming yields AgentStreamResponse chunks + final_message
• test_process_request_with_retriever — retriever called, context appended
• test_process_request_api_error — exception propagation
• test_is_streaming_enabled — False default, True when set

Integration test (requires credentials):

• test_gemini_vertex_ai_integration — end-to-end via Vertex AI with GOOGLE_APPLICATION_CREDENTIALS

Run integration test with:

GOOGLE_APPLICATION_CREDENTIALS=/path/to/credentials.json pytest -m integration python/src/tests/agents/test_gemini_agent.py -v

Test plan

• from agent_squad.agents import GeminiAgent works with google-genai installed
• Import doesn't break when google-genai is NOT installed
• All 8 unit tests pass (pytest -v -m "not integration")
• Integration test with valid Vertex AI project

Fixes #376