[Generated documents] Add durable long-form Word document generation

[paullizer]

Summary

Add durable long-form Word document generation so a user, agent, or workflow can request a new DOCX from conversation, selected-source, or source-free evidence and receive an authorized downloadable artifact without relying on the synchronous single-message export route.

This is generation of a new document, not modification of an existing Office file or template population.

User Value

The current Word export is useful for a short message, but users also need complete deliverables such as five-, 20-, or 50-page reports, proposals, briefs, plans, and analysis packages. Those jobs can exceed normal request timeouts, message-size assumptions, and in-memory rendering expectations.

Current Limitation

/api/message/export-word renders one message synchronously and returns an immediate browser download. It has no durable job record, checkpointing, cancellation, resume/retry semantics, progress reporting, or artifact publication lifecycle for large multi-section document generation.

Proposed Behavior

• Build a new DOCX from a natural-language request through Chat, Search, selected agents, Analyze, Compare, workflows, or source-free prompts.
• Use a hierarchical document plan: validated outline, section-level content work, deterministic DOCX rendering, and final package validation.
• Publish the completed document through the existing authorized generated-chat-artifact/download/promotion flow.
• Use a generic durable generated-artifact job contract for queued/running/completed/failed/canceled state, leases, checkpoints, idempotent publication, cancellation, retry, and reauthorization.
• Coordinate with [CSV exports] Support universal CSV generation across all paths and file types #1071 so the durable job and artifact-publication primitives are format-neutral; do not extend the CSV/tabular executor directly.
• Keep DOCX planning and rendering format-specific, using the existing Word-native formatting helpers as a starting point.
• Preserve normal immediate export for small single-message exports where appropriate.

Acceptance Criteria

• Users can request a new DOCX through the supported generation paths rather than needing to first export an existing assistant message.
• A long-form request can produce at least 5, 20, and 50-page-equivalent document workloads within configured safety and size limits.
• Generation uses a persisted outline/section plan and checkpointed section progress rather than one unbounded model response.
• The job exposes queued, running, completed, failed, and canceled states; supports authorized cancellation, retry/resume where safe, and progress visible from the originating conversation.
• Source access, source revision, and conversation authorization are revalidated at execution and before artifact publication.
• DOCX output renders headings, paragraphs, lists, tables, code, citations, images/charts where supported, and a configurable document structure without silently dropping planned sections.
• Rendering failures, unsupported content, source coverage gaps, and partial documents are explicit and never published as a misleading complete report.
• Completion creates one idempotently published, authorized downloadable chat artifact with existing workspace promotion support.
• Small existing per-message Word export remains compatible and does not require a background job.
• Functional tests cover short/long generation, multi-section checkpoints, cancellation, retry, authorization loss, source version change, artifact download, duplicate-publication prevention, and representative DOCX round-trip validation.

Non-Goals

• Editing an existing Office file, tracked by Design Office file modification workspace #969 and Develop reusable framework for conversation-linked visual file editing #999.
• Template selection or named-field population, tracked by Support Word and PowerPoint templates for generated files #968 and Populate DOCX and fillable PDF templates from agent or workflow analysis #1045.
• Reusing the tabular CSV/JSON batch executor as the DOCX renderer.

Related Work

• [CSV exports] Support universal CSV generation across all paths and file types #1071 establishes the universal CSV output capability and should drive extraction of format-neutral durable artifact/job primitives.
• generate word, ppt, excel, and csv as outputs #482 delivered the original basic Word/PowerPoint/Excel/CSV output capability; this issue extends it to large, durable DOCX generation.