Fix @fedify/backfill not follow collection pages during conversation backfill

[sij411]

Follow first and next links in context and replies collections so paginated conversations yield all available posts. Load pages lazily through the shared request controls, stop page cycles, and preserve items already yielded when a later page fails.

Add regression coverage for pagination, limits, cancellation, and page failures, and document the behaviour.

Fixes #1248

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📝 Walkthrough

Walkthrough

Backfill now follows first and successive next pages in context and replies collections. It applies traversal limits, stops repeated-page traversal, and retains items yielded before a page-load failure. Tests and documentation cover these behaviors.

Changes

Backfill Collection Pagination

Layer / File(s)	Summary
Collection page traversal packages/backfill/src/backfill.ts	getCollectionItems now traverses first and next pages. It stops on repeated pages, handles cancellation, and ends traversal on page-load errors while propagating request-limit errors.
Pagination validation and documentation packages/backfill/src/backfill.test.ts, packages/backfill/README.md, changes.d/backfill/follow-collection-pages.md, CHANGES.md	Tests cover page types, traversal limits, cancellation, caching, and page-load failures. The README and changelog describe pagination and request-limit behavior.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant Backfill
  participant getCollectionItems
  participant documentLoader
  Backfill->>getCollectionItems: Traverse a context or replies collection
  getCollectionItems->>documentLoader: Request a first or next page
  documentLoader-->>getCollectionItems: Return the page
  getCollectionItems-->>Backfill: Yield page items

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Merge Risk: 🔵 Low · up to f6b4e

The pagination change is mergeable once the direct changelog edit is removed; the fragment should remain as the release entry.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (3 skipped: 3 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue #1248 requires pagination for context and replies collections, lazy page loading, shared traversal controls, cycle detection, no added reply-tree depth, and retention of yielded items after a pa…
Out of Scope Changes check	✅ Passed	The source change, regression tests, README update, and changelog entries all implement or document issue #1248. The reviewed change summary shows no unrelated changes.
Title check	✅ Passed	The title clearly identifies the collection-page pagination fix in @fedify/backfill.
Description check	✅ Passed	The description explains the pagination behavior, request controls, failure handling, tests, and documentation changes.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (3 skipped: 3 unsupported.)

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Actionable comments posted: 1

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Inline comments:
Review comments at @CHANGES.md:
- Around line 11-19: Remove the unreleased @fedify/backfill entry from
CHANGES.md because it duplicates the existing release fragment; leave the
fragment as the sole source for this changelog entry.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between ffa5f1f and f6b4eab.

📒 Files selected for processing (5)

• CHANGES.md
• changes.d/backfill/follow-collection-pages.md
• packages/backfill/README.md
• packages/backfill/src/backfill.test.ts
• packages/backfill/src/backfill.ts

Included review availability: This review used your included allowance. Your plan provides up to 4 included reviews per hour; 3 remain after this review.

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
packages/backfill/src/backfill.ts	96.29%	0 Missing and 2 partials ⚠️

Files with missing lines	Coverage Δ
packages/backfill/src/backfill.ts	90.86% <96.29%> (+0.64%)	⬆️

... and 3 files with indirect coverage changes

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