Fix schema string output coercion

[DavidBakerEffendi]

Description

Adds deterministic repair for schema-backed outputs when models return recoverable string-field shapes, such as prose arrays or scalar values where the response schema expects a non-enum string. Strict validation still runs after coercion so unrecoverable shapes continue to fail.

Key Changes:

• Add shared schema-output coercion for non-enum string fields, including array/scalar repair and validated-only acceptance.
• Apply the coercion path to job-level direct answers and schema-aware custom-agent terminal/repair outputs.
• Add focused unit and integration coverage for array-to-string, scalar-to-string, enum protection, and JSON object/null boundaries.

Touch Points:

• app/src/main/java/ai/brokk/executor/jobs/JobResponseSchemaSupport.java
• app/src/main/java/ai/brokk/executor/jobs/JobRunner.java
• app/src/main/java/ai/brokk/executor/agents/CustomAgentExecutor.java
• app/src/test/java/ai/brokk/executor/jobs/JobResponseSchemaSupportTest.java
• app/src/test/java/ai/brokk/executor/jobs/JobRunnerResponseSchemaTest.java
• app/src/test/java/ai/brokk/executor/agents/CustomAgentExecutorTest.java

Validation:

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