Salvaged from PR #1292 onto current main. Preserve per-job model,
provider, and base_url overrides in cron execution, persist them in
job records, expose them through the cronjob tool create/update paths,
and add regression coverage. Deliberately does not persist per-job
api_key values.
Cover model_tools, toolset_distributions, context_compressor,
prompt_caching, cronjob_tools, session_search, process_registry,
and cron/scheduler with 127 new test cases.
