Enforce schema validation on headless structured outputs

[DavidBakerEffendi]

Summary

• Add post-generation validation for schema-backed outputs in JobRunner and CustomAgentExecutor so malformed JSON now fails instead of being treated as success.
• Keep the existing provider-side response format request, but surface invalid outputs as RESPONSE_SCHEMA_OUTPUT_INVALID with a hard failure path.
• Extend tests to cover invalid direct answers, invalid custom-agent finals, and the retry flow for schema-backed custom-agent output.

Testing

• ./gradlew :app:test --tests ai.brokk.executor.jobs.JobRunnerResponseSchemaTest --tests ai.brokk.executor.agents.CustomAgentExecutorTest