Add universal CSV generation across response paths

[Paul Lizer (paullizer)]

Summary

• Unifies CSV intent detection and artifact finalization across standard and streaming Chat, Chat Search, selected agents, Analyze, Compare, workflows, and source-free structured responses.
• Adds one conversation-persisted row/schema clarification for ambiguous CSV requests; explicit row and column instructions proceed directly.
• Reuses the authorized immediate and durable background artifact contracts, including staged-row authorization, cancellation, reauthorization, and idempotent publication.
• Removes the default-off mixed-source Analyze gate so selected narrative and tabular sources consistently use the native combined workflow for model and agent runners.
• Updates versioned functional coverage, documentation, and release notes for v0.250.071.

Validation

• python functional_tests/test_assistant_table_csv_artifact.py - 27/27 passed
• python functional_tests/test_mixed_source_analyze_workflow.py - passed
• python functional_tests/test_document_action_token_usage_aggregation.py - 5/5 passed
• python functional_tests/test_document_action_conversation_scope_metadata.py - 4/4 passed
• python functional_tests/test_mixed_source_conversation_continuity.py - 3/3 passed
• python functional_tests/test_tabular_row_orchestration_scale.py - passed
• python functional_tests/test_tabular_background_generated_exports.py - 7/7 passed
• python functional_tests/test_document_analysis_generated_artifacts.py - 2/2 passed
• python functional_tests/test_document_comparison_generated_artifacts.py - 2/2 passed
• python -m py_compile on modified production modules - passed
• Editor diagnostics and whitespace checks - passed

Notes

• The branch is rebased on the current target, which already includes PR Harden mixed-source orchestration and CSV exports #1074's CSV timeout and throughput hardening.

Refs #1071
Refs #1058