fix(git): stop opting into raise_exceptions=True

[whyujjwal]

Summary

• stop passing raise_exceptions=True when starting the Python git server
• align git server behavior with sibling Python servers so JSON-RPC validation errors are returned to the client instead of crashing the process
• add a regression test covering the serve() startup path

Testing

• uv run pytest tests/test_server.py -q
• uv run ruff check tests/test_server.py src/mcp_server_git/server.py
• uv run pyright

Closes #4213

[LuuOW]

Technical audit: Verified MCP server implementation for consistency with current SDK patterns.