fix(ai): clarify stale proposal reanchor conflicts

[iipanda]

Summary

• Return source-anchor conflict messages when stale replace proposals cannot reanchor.
• Reuse replace-selection match validation for stale revision fallback.
• Add regression coverage for missing source text.

Test Plan

• bun test --cwd packages/modules/core.ai ./src/server/routes.test.ts
• bun run format:check
• bun run changeset:check
• bun run check

Summary by CodeRabbit

• Bug Fixes

  • Enhanced conflict detection when applying AI proposals with replace-selection operations on stale drafts, now providing more specific error messages when original text cannot be re-anchored.

• Tests

  • Added test coverage validating proper error handling for stale replace-selection proposals.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 549743a1-b110-4420-8634-97f3fa810430

📥 Commits

Reviewing files that changed from the base of the PR and between a5716ee and 2eaf8ef.

📒 Files selected for processing (2)

• packages/modules/core.ai/src/server/apply.ts
• packages/modules/core.ai/src/server/routes.test.ts

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📝 Walkthrough

Walkthrough

This PR improves error handling for AI proposal applications by extracting selection-matching logic into a reusable helper and prioritizing specific conflict errors. When replace_selection proposals become stale and cannot locate their original text in the current draft, the system now signals a selection-specific conflict instead of a generic base-draft mismatch.

Changes

Replace Selection Conflict Detection

Layer / File(s)	Summary
Helper type and conflict detection packages/modules/core.ai/src/server/apply.ts	Introduces ReplaceSelectionOperation type alias and findReplaceSelectionMatch helper that locates the unique occurrence of operation.originalText in a draft body, routing match failures to aiProposalConflict with selection-specific error details.
Apply operation integration packages/modules/core.ai/src/server/apply.ts	applyReplaceSelection now uses the helper to compute replacement position. applyAiProposal adds an intermediate call to the helper in the base-draft mismatch failure path for replace_selection proposals, ensuring specific conflict conditions trigger before the generic mismatch error.
Test coverage for stale selection conflicts packages/modules/core.ai/src/server/routes.test.ts	New route test verifies that stale replace_selection proposals fail with 409 AI_PROPOSAL_CONFLICT when the original selection text cannot be re-anchored in the current draft, asserting the error response includes selection-specific details and confirms no document update occurs.

Sequence Diagram

sequenceDiagram
  participant Client
  participant applyAiProposal
  participant findReplaceSelectionMatch
  participant applyReplaceSelection
  participant aiProposalConflict
  Client->>applyAiProposal: POST /api/v1/ai/proposals/:id/apply
  alt baseDraftRevision matches
    applyAiProposal->>applyReplaceSelection: call with operation
    applyReplaceSelection->>findReplaceSelectionMatch: locate originalText
    alt originalText found uniquely
      findReplaceSelectionMatch-->>applyReplaceSelection: return match index
      applyReplaceSelection->>applyReplaceSelection: update body at index
      applyReplaceSelection-->>applyAiProposal: success
      applyAiProposal-->>Client: 200 OK
    else originalText missing or ambiguous
      findReplaceSelectionMatch-->>aiProposalConflict: emit selection details
      aiProposalConflict-->>applyAiProposal: conflict error
      applyAiProposal-->>Client: 409 AI_PROPOSAL_CONFLICT
    end
  else baseDraftRevision does not match
    applyAiProposal->>findReplaceSelectionMatch: check if replace_selection still valid
    alt originalText found uniquely
      findReplaceSelectionMatch-->>applyAiProposal: match index
      applyAiProposal->>applyReplaceSelection: proceed with stale revision
      applyReplaceSelection-->>applyAiProposal: success
      applyAiProposal-->>Client: 200 OK
    else originalText missing or ambiguous
      findReplaceSelectionMatch-->>aiProposalConflict: emit selection details
      aiProposalConflict-->>applyAiProposal: selection conflict error
      applyAiProposal-->>Client: 409 AI_PROPOSAL_CONFLICT
    end
  end

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Estimated Code Review Effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

A selection once made, now must stand its ground,
When drafts grow stale and sources cannot be found,
A rabbit refined the detection with care,
Specific conflicts now float through the air,
No more generic errors to spare! 🐰✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title accurately describes the main change: it addresses how stale AI proposals handle reanchoring conflicts by clarifying the error messages returned.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/ai-proposal-reanchor-conflict

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