#3012 fix(auth): match complete WWW-Authenticate parameters

Summary

• match WWW-Authenticate auth-param names only at parameter boundaries
• prevent suffix matches such as error_scope satisfying scope
• add regressions for scope and resource_metadata prefixed parameters

Test plan

• uv run pytest tests/client/test_auth.py::TestWWWAuthenticate -q
• uv run ruff check src/mcp/client/auth/utils.py tests/client/test_auth.py
• uv run pyright src/mcp/client/auth/utils.py tests/client/test_auth.py
• git diff --check

Related: https://github.com/modelcontextprotocol/python-sdk/issues/2902