fix: normalize oauth redirect uri url types

[he-yufeng]

Summary

• normalize OAuth redirect URI URL objects at the client metadata model boundary
• keep validation and serialization unchanged while allowing AnyUrl/AnyHttpUrl subtype round-trips to compare correctly
• add a shared auth regression test for an AnyHttpUrl registration URI checked against an incoming AnyUrl

Fixes #2687

To verify

• uv run pytest tests/shared/test_auth.py -q
• uv run ruff check src/mcp/shared/auth.py tests/shared/test_auth.py
• uv run ruff format --check src/mcp/shared/auth.py tests/shared/test_auth.py
• uv run pyright src/mcp/shared/auth.py tests/shared/test_auth.py
• git diff --check