feat: expose schema generator for tool schemas

[MukundaKatta]

Summary

• add an optional schema_generator parameter to MCPServer and pass it through tool registration
• use the custom Pydantic JSON schema generator for tool input schemas and structured output schemas
• add regression coverage for custom primitive union formatting and default anyOf behavior

Fixes #2582

Tests

• uv run ruff check src/mcp/server/mcpserver/server.py src/mcp/server/mcpserver/tools/tool_manager.py src/mcp/server/mcpserver/tools/base.py src/mcp/server/mcpserver/utilities/func_metadata.py tests/server/mcpserver/test_server.py
• uv run pytest tests/server/mcpserver/test_server.py tests/server/mcpserver/test_tool_manager.py tests/server/mcpserver/tools/test_base.py tests/server/mcpserver/test_func_metadata.py -q

[maxisbey]

Thanks for the PR. Per our CONTRIBUTING.md we ask that PRs start from a linked, triaged issue with maintainer buy-in, which this one doesn't have yet. I'm closing this out as part of a backlog cleanup around the v2 release. Feel free to reopen if this is still relevant.

_{AI Disclaimer}