Enable custom exception types for workflow failures

[cdgamarose-nv]

Custom exception types for workflow failures.
In @src/aiq_agent/agents/deep_researcher/agent.py

Lines 215 and 234 raise ValueError for runtime workflow failures (no markdown output, no valid citations). These are not programming errors but legitimate failure modes of the LLM-driven workflow. Custom exception types (e.g., WorkflowOutputError, CitationValidationError) would:

1. Allow callers to distinguish workflow failures from actual programming errors
2. Enable more precise error handling and retry logic at higher layers
3. Provide clearer semantics than generic ValueError

Note that line 243 already uses the custom EmptySourceRegistryError for similar conditions.

Also applies to: 233-234

Originally posted by @coderabbitai[bot] in #267 (comment)