fix: preserve OAuth authorization endpoint query params

[Zhang-986]

Summary

• preserve existing query parameters on OAuth authorization endpoints when building the redirect URL
• append SDK-generated authorization request parameters with proper URL query encoding
• add a regression test for authorization endpoints such as /authorize?prompt=select_account

Fixes #2776.

Tests

• uv run --frozen pytest tests/client/test_auth.py -k authorization_endpoint_query_params_are_preserved (fails before the fix, passes after)
• uv run --frozen pytest tests/client/test_auth.py
• uv run --frozen ruff check src/mcp/client/auth/oauth2.py tests/client/test_auth.py
• uv run --frozen pyright src/mcp/client/auth/oauth2.py tests/client/test_auth.py
• git diff --check

[Kludex]

You've opened a duplicated pull request, please search opened PRs before creating new ones. Duplicated from #2779.