#19774: Allow explicit model allowlist entries missing from provider catalogs

## Summary - treat exact entries in `agents.defaults.models` as authoritative allowlist keys even when provider discovery catalogs don't list them (e.g. Bedrock inference profile IDs) - keep session and subagent model overrides stable for explicitly allowlisted catalog-missing models instead of silently resetting to primary - add regression coverage across model-selection core logic, reply model inheritance, session patching, and subagent spawn defaults ## Testing - targeted test execution was not possible in this environment (`pnpm` is unavailable) Fixes #19460