fix(flows): make paper_flow produce a valid Paper under strict structured output

[savycompany]

Description

paper_flow currently cannot complete an end-to-end extraction against the OpenAI Agents SDK, because it sets the agent's output_type to the canonical knowledge.Paper — a store schema that is hostile to structured output:

• Paper.nodes is a dict[UUID, TreeNode]. A dict serialises to additionalProperties, which the SDK's strict JSON-schema mode rejects:
  UserError: additionalProperties should not be set for object types.
• With strict mode off, models don't emit RFC-4122 UUIDs, so the UUID id fields fail Pydantic validation on free-form ids like "intro_node":
  Input should be a valid UUID ... 'root'.

Either way the default flow raises before returning — no real paper extracts. This is the migration gap flagged in the README note (#71).

Approach

Keep the canonical store schema untouched — its UUID identity + flat dict map are load-bearing for dedup/indexing per docs/design/en/storage.md — and fix the issue at the apex (flows) layer, which is where the LLM boundary actually lives (and the only layer import-linter allows to import knowledge):

• quantmind/flows/_paper_draft.py (new): PaperDraft / PaperDraftNode, a strict-schema-safe extraction target. Children are embedded (a real nested tree) rather than referenced by id, so the model never maintains a flat id map — eliminating dangling-reference / duplicate-id / missing-root failure modes. draft_to_paper() lifts a validated draft into a canonical Paper: assigns real UUIDs, wires parent_id / children_ids / position, and injects provenance (source, arxiv_id, authors, publication-date as_of) the flow already knows from the fetch layer instead of asking the model to author it.
• quantmind/flows/paper.py: targets PaperDraft by default and lifts the result into a Paper. A caller-supplied output_type still bypasses the draft and is returned verbatim (isinstance(result, Paper) pass-through), preserving the existing override contract. The arxiv branch of _fetch_and_format now propagates published_at so as_of is the real cutoff.

As a side effect this also fixes empty Paper.arxiv_id / Paper.authors: previously the model was asked to author provenance and routinely left it blank.

Verification

• Full golden harness green: ruff format / ruff check / basedpyright / lint-imports / pytest — 248 passed, 90% coverage (15 new unit tests for the draft schema, converter, provenance injection, and the flow integration path).
• End-to-end against the live Agents SDK (gpt-4o-mini, default path, no strict-schema overrides): a real arXiv paper (2605.20636) extracts into a 19-node Paper with UUID ids and a correct section/subsection tree, then survives a JSON store round-trip.

Checklist

• The PR title starts with $CATEGORY(xx): xxx (fix(flows): ...)
• Related issue is referred in this PR — relates to the SDK migration (Tracking: Migrate Agent layer to OpenAI Agents SDK #71); happy to link a dedicated issue if preferred
• The markdown and latex are rendered correctly.
• The code in PR is well-documented.
• The PR is complete and small (4 files, +427/-11)

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