fix(client): preserve OAuth endpoint query params

[he-yufeng]

Summary

• preserve existing query parameters on the authorization endpoint when adding OAuth parameters
• cover the Salesforce-style endpoint from OAuth handler doesn't support redirect URLs with params #2776 with a regression test
• keep the existing client id/scope handling type-safe for pyright

Fixes #2776.

To verify

• uv run --frozen pytest tests/client/test_auth.py::TestOAuthFallback::test_authorization_endpoint_preserves_existing_query_params -q
• uv run --frozen pytest tests/client/test_auth.py -q
• uv run --frozen pyright src/mcp/client/auth/oauth2.py tests/client/test_auth.py
• uv run --frozen ruff check src/mcp/client/auth/oauth2.py tests/client/test_auth.py
• git diff --check

[he-yufeng]

This is the kept OAuth endpoint query-parameter branch after closing the older duplicate #2787.

The current check matrix is green across pre-commit, conformance, readme snippets, and Python 3.10-3.14 on Ubuntu/Windows.

Could a maintainer review when convenient?