fix: raise PRSplitError for chunking failures

[vitali87]

Problem

chunk_hunks_greedy / chunk_hunks_dynamic_programming raise a bare ValueError when a single hunk's token estimate exceeds the per-chunk budget (ErrorMsg.HUNK_TOO_LARGE), and chunk_hunks does the same for an unknown strategy. Neither is a PRSplitError, so on a huge diff with one giant hunk the split command dies with a traceback instead of the one-line message every other planner failure gets (and #63's planner error boundary can't catch it either).

Fix

All four raises in pr_split/planner/chunker.py now use PRSplitError (the messages are unchanged and already come from ErrorMsg). Tests updated and a new test asserts both strategies raise PRSplitError with the file/hunk/budget in the message.

445 tests pass, ruff clean.

Summary by CodeRabbit

• Bug Fixes
  • Chunking failures now consistently report the application’s standard error type.
  • Improved consistency for oversized changes, unsupported chunking strategies, and reconstruction failures.

[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: f11c52e0-6d78-4622-980a-239049f193e7

📥 Commits

Reviewing files that changed from the base of the PR and between e8e81d6 and ba17cb2.

📒 Files selected for processing (2)

• pr_split/planner/chunker.py
• tests/test_chunker.py

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

Chunking error paths now raise PRSplitError instead of ValueError. Tests verify this behavior for oversized hunks in both strategies, dynamic-programming reconstruction failures, and unknown strategies.

Changes

Chunking error handling

Layer / File(s)	Summary
Use project-specific chunking errors pr_split/planner/chunker.py	The chunker raises PRSplitError for oversized hunks, failed dynamic-programming reconstruction, and unknown strategies.
Verify exception behavior tests/test_chunker.py	Existing expectations now use PRSplitError. New coverage checks oversized hunks for greedy and dynamic-programming strategies.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to ba17c

Chunking failures now use the project’s standard error type, so oversized hunks and unknown strategies produce the intended concise error instead of an uncaught traceback. No actionable merge-blocking risk remains after normal checks and review.

🚥 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 9 functions across 2 files.	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 title clearly and concisely describes the main change: chunking failures now raise the project-specific PRSplitError.
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.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/chunker-errors-are-pr-split-errors

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