feat: add optional per-agent Vertex AI project and location configuration

[avidela]

Summary

Adds optional per agent project and location configuration for Vertex AI models ( Claude and Gemini)

Addresses issues:

1. Support for specifying GCP Project and Location per Vertex AI Model instance #485
2. Region needs to be configurable on every model #1120
3. Allow users to define GOOGLE_CLOUD_PROJECT, GOOGLE_CLOUD_LOCATION, GOOGLE_GENAI_USE_VERTEXAI in code #1417

Changes

• Add optional project_id and location fields to Claude and Gemini classes
• Enable per-instance client caching based on configuration
• Maintain backward compatibility with environment variables
• Add comprehensive test coverage

Testing Plan

Unit Tests

• Added comprehensive unit tests in tests/unittests/models/test_vertex_per_agent_config.py
• All existing tests pass
• New tests cover: custom config, per-instance config, backward compatibility, mixed scenarios

E2E Testing

Creating repository https://github.com/avidela/adkfeat-add-project-location-config-to-vertex-models where I installed the wheel from this branch, and ran a normal multi_tool_agent and then a multi project and location agent, where I tested different projects and different regions in an agent -> sub_agents manner and it works.

Backward Compatibility

✅ Existing code using environment variables continues to work unchanged
✅ No breaking changes to existing APIs