fix: preserve query params in OAuth authorization endpoint

[he-yufeng]

Summary

Fixes #2776.

Some OAuth authorization server metadata includes an authorization_endpoint that already has query parameters. The client was building the redirect URL with a raw ?{urlencode(...)} append, which turns endpoints like:

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

into a malformed URL with a second question mark before the OAuth request parameters.

This uses httpx.URL.copy_merge_params() to add the OAuth request parameters through the URL parser instead of string concatenation, preserving existing endpoint query parameters.

Verification

uv run --no-sync pytest tests\client\test_auth.py::TestAuthFlow::test_authorization_endpoint_preserves_existing_query_params -q --basetemp .tmp\pytest-2776-one -p no:cacheprovider
uv run --no-sync pytest tests\client\test_auth.py -q --basetemp .tmp\pytest-2776-auth -p no:cacheprovider
uv run --no-sync ruff check --no-cache src\mcp\client\auth\oauth2.py tests\client\test_auth.py
uv run --no-sync ruff format --check --no-cache src\mcp\client\auth\oauth2.py tests\client\test_auth.py
uv run --no-sync pyright src\mcp\client\auth\oauth2.py tests\client\test_auth.py
git diff --check

[he-yufeng]

Follow-up for the lowest-direct CI failure:

• The failure was an existing test assertion that expected spaces in the scope query parameter to be encoded as +.
• httpx.URL.copy_merge_params() can encode spaces as %20 with the lowest direct dependency set, which is still valid URL encoding.
• I changed that assertion to parse the URL query and compare the decoded scope value instead.

Re-verified:

uv run --no-sync pytest tests\client\test_auth.py::TestAuthFlow::test_authorization_endpoint_preserves_existing_query_params tests\client\test_auth.py::TestAuthFlow::test_403_insufficient_scope_updates_scope_from_header -q --basetemp .tmp\pytest-2781-two -p no:cacheprovider
uv run --no-sync pytest tests\client\test_auth.py -q --basetemp .tmp\pytest-2781-auth2 -p no:cacheprovider
uv run --no-sync ruff check --no-cache src\mcp\client\auth\oauth2.py tests\client\test_auth.py
uv run --no-sync ruff format --check --no-cache src\mcp\client\auth\oauth2.py tests\client\test_auth.py
uv run --no-sync pyright src\mcp\client\auth\oauth2.py tests\client\test_auth.py
git diff --check

[he-yufeng]

Closing this in favor of #2779, which already covers the same #2776 OAuth authorization endpoint query handling with a broader test matrix. I missed that duplicate before opening this PR.