fix(runs): resolve run URLs for custom API base URLs

[Yash1hi]

Summary

run_and_evaluate, async_run_and_evaluate, and the other run helpers returned a staging app link whenever the API base URL didn't exactly match one of the built-in environments, for example a proxy in front of production or a local backend on another port. Run links now match the production, staging, and local environments after normalizing the URL, fall back to the production app for unrecognized hosts, and honor a new SCORECARD_APP_URL override for dedicated deployments.

Changes

• src/scorecard_ai/lib/_helpers.py: resolve the app URL from the three ENVIRONMENTS entries (trailing slashes and scheme/host case ignored); unknown hosts use https://app.scorecard.io; SCORECARD_APP_URL overrides it, minus trailing slashes.
• tests/lib/test_run_url.py: offline tests for each environment, normalization, unknown hosts, the override, and every run helper.
• The generated client's base_app_url property is unchanged; it still falls back to staging for unknown hosts and lives in Stainless-managed code.

The TypeScript SDK gets the same rules in scorecard-ai/scorecard-node#51.

Testing

• .venv/bin/python -m pytest -q tests/lib/test_run_url.py: 55 passed.
• ruff check, ruff format --check, and pyright on the touched files: passed.