Add generated file export framework

[Paul Lizer (paullizer)]

Summary

• Adds a format-neutral generated file export framework for CSV, DOCX, and PDF across standard and streaming Chat, agents, actions, Analyze, Compare, workflows, and source-free responses.
• Promotes successful current-turn structured function results into the same authorized artifact pipeline as final assistant content, with secret-like fields excluded and multi-action source provenance retained.
• Preserves the existing durable CSV queue, checkpoints, cancellation, reauthorization, formula protection, and coverage-aware tabular export path.
• Adds immediate DOCX and PDF rendering for response-sized generated files; long-form durable document generation remains scoped to [Generated documents] Add durable long-form Word document generation #1072 and [Generated presentations] Add durable long-form PowerPoint generation #1073.
• Expands bounded authorized retrieval defaults for search, metadata extraction, directory lookup, assigned knowledge, workflows, and mixed-source tabular candidate discovery.

Validation

• python functional_tests/test_assistant_table_csv_artifact.py - 31/31 passed
• python functional_tests/test_mixed_source_manifest_contracts.py - 13/13 passed
• python functional_tests/test_document_action_token_usage_aggregation.py - 5/5 passed
• python -m py_compile on changed production modules - passed
• Editor diagnostics and whitespace checks - passed

Known Test Note

• functional_tests/test_document_search_api_and_plugin.py validates the updated search helper contract but still fails its pre-existing assertion that app.py registers route_backend_search; that registration is outside this change.

Refs #1071
Refs #1072
Refs #1073