fix(auth): preserve authorization endpoint query params

[go165]

Summary

• build OAuth authorization URLs by merging generated OAuth params into the discovered authorization endpoint query instead of always appending a new ?
• preserve provider-required endpoint parameters such as prompt=select_account
• add an interaction regression test for an authorization endpoint that already contains query params

Fixes #2776

Validation

• .venv/bin/python -m pytest tests/interaction/auth/test_authorize_token.py
• .venv/bin/python -m ruff check src/mcp/client/auth/oauth2.py tests/interaction/auth/test_authorize_token.py
• .venv/bin/python -m ruff format --check src/mcp/client/auth/oauth2.py tests/interaction/auth/test_authorize_token.py
• git diff --check