fix(stack): add descriptive message to PullRequestNotExistError

[jd]

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

[jd]

This pull request is part of a stack:

1. fix(stack): add descriptive message to PullRequestNotExistError (#1158) 👈
2. fix(stack): close backtick in branch target error message (#1159)
3. fix(stack): use asyncio.gather instead of asyncio.wait for orphan deletion (#1160)

[mergify]

Merge Protections

Your pull request matches the following merge protections and will not be merged until they are valid.

🟢 🤖 Continuous Integration

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• all of:
  • check-success=ci-gate

🟢 👀 Review Requirements

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• any of:
  • #approved-reviews-by>=2
  • author = dependabot[bot]
  • author = mergify-ci-bot
  • author = renovate[bot]

🟢 Enforce conventional commit

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Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?:

🟢 🔎 Reviews

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• #changes-requested-reviews-by = 0
• #review-requested = 0
• #review-threads-unresolved = 0

🟢 📕 PR description

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• body ~= (?ms:.{48,})

[Copilot]

Pull request overview

This PR improves the usability of stack change handling by providing a descriptive error message when a Change instance has no associated pull request.

Changes:

• Add a human-readable message when raising PullRequestNotExistError from Change.pull_head_sha.

---

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[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-04-15 06:50 UTC · Rule: default
• ✅ Checks passed · on draft merge queue: embarking main (213e9fe) and #1158 together #1162
• ✅ Merged — 2026-04-15 06:56 UTC · at 6ee2be6eb13175c152a13ce1dc583c4a99e04278

This pull request spent 5 minutes 59 seconds in the queue, including 4 minutes 48 seconds running CI.

Required conditions to merge

• all of [🛡 Merge Protections rule Enforce conventional commit]:
  • title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?:
    • fix(stack): add descriptive message to PullRequestNotExistError #1158
• all of [🛡 Merge Protections rule 👀 Review Requirements]:
  • any of:
    • #approved-reviews-by>=2
      • fix(stack): add descriptive message to PullRequestNotExistError #1158
    • author = dependabot[bot]
      • fix(stack): add descriptive message to PullRequestNotExistError #1158
    • author = mergify-ci-bot
      • fix(stack): add descriptive message to PullRequestNotExistError #1158
    • author = renovate[bot]
      • fix(stack): add descriptive message to PullRequestNotExistError #1158
• all of [🛡 Merge Protections rule 📕 PR description]:
  • body ~= (?ms:.{48,})
    • fix(stack): add descriptive message to PullRequestNotExistError #1158
• all of [🛡 Merge Protections rule 🔎 Reviews]:
  • #changes-requested-reviews-by = 0
    • fix(stack): add descriptive message to PullRequestNotExistError #1158
  • #review-requested = 0
    • fix(stack): add descriptive message to PullRequestNotExistError #1158
  • #review-threads-unresolved = 0
    • fix(stack): add descriptive message to PullRequestNotExistError #1158
• all of [🛡 Merge Protections rule 🤖 Continuous Integration]:
  • all of:
    • check-success=ci-gate