Fixes #13288.

Root cause:
Gateway/Weixin-created cron jobs could be created without explicit runtime provider fields, leaving the persisted job to fall back to the invalid legacy openai provider at execution time. Equivalent CLI-created jobs had a working active runtime provider available.

Fix summary:

• Fill missing cron create provider and base_url from resolve_runtime_provider() before persisting the job.
• Preserve explicit cron provider / base_url values when supplied.
• Keep failures non-fatal: if runtime resolution is unavailable, creation falls back to the previous optional values.
• Add a Weixin/gateway-session regression proving the stored job inherits the active runtime provider/base URL instead of openai.

Tests:

• uv run --frozen --python 3.11 --extra dev pytest -o addopts= tests/tools/test_cronjob_tools.py -q -> 26 passed
• git diff --check -- tools/cronjob_tools.py tests/tools/test_cronjob_tools.py