fix: preserve OAuth authorization endpoint query params

[he-yufeng]

Fixes #2776.

Summary

The OAuth authorization URL builder was always appending SDK parameters with a literal ?:

f"{auth_endpoint}?{urlencode(auth_params)}"

That breaks authorization servers whose metadata already includes endpoint-level query parameters. Salesforce's sandbox authorization endpoint is one example:

https://test.salesforce.com/services/oauth2/authorize?prompt=select_account

The generated URL became ...?prompt=select_account?response_type=..., so the OAuth parameters were folded into the existing prompt value instead of being sent as query parameters.

This change parses the endpoint query first, preserves existing parameters, and appends the SDK-generated OAuth parameters with normal form encoding.

Validation

python -m py_compile src\mcp\client\auth\oauth2.py tests\client\test_auth.py
$env:PYTHONPATH='C:\dev\GITHUB-clean\python-sdk\src'
python -m pytest tests\client\test_auth.py -q -k "authorization_endpoint_preserves_existing_query or auth_flow_adds_offline_access_when_as_advertises"
python -m ruff check src\mcp\client\auth\oauth2.py tests\client\test_auth.py
git diff --check upstream/main...HEAD