[split-1066] U6 - fix(write-to-file): run the diff cleanup when handleError rejects #1937

Unit U6 of the PR #1066 split (5/6).

Split plan: #703
PR: #1932
Content source of record: tag pr1066-source = 46d1d218701f0ce2d675b1b315489bacb6b0f77d (easonLiangWorldedtech/Zoo-Code).

Unit contract

The execute() cleanup invariant: a finally around handleError so the diff cleanup runs even when handleError rejects, the writeApproved flag, and the consecutive-mistake-counter order.

Why this unit exists on its own

Single provider group + single gate scope (the execute() error path).

Boundary

• base: e3c10401f760 (previous unit head, U3)
• head: 9b93a6f882a4
• chain: U3 -> U6 -> U7
• merge order is fixed: U12 -> U4 -> U5 -> U3 -> U6 -> U7/FINAL; the FINAL integration PR (fix(write-to-file): clean partial state on missing-param and rooignore denial + integrate the #1066 split series (6/6 + FINAL) #1928) is the sole merge target.

Files and budget

• src/core/tools/WriteToFileTool.ts +45/-11 — content subset of source
• src/core/tools/__tests__/writeToFileTool.spec.ts +379/-4 — content subset of source
• budget: 439 a+d / 2 files — SOFT-OVERSHOOT (rationale: single provider group + single gate scope; under the 1000 hard cap)
• mutation gate: 34 changed executable lines — under the 500 cap; valid mutants for the whole PR are 116 / 400, so no directive was added by this unit.

Verification (must pass by once, binary)

• zdt split verify --contract U6.json --worktree <wt> --head 9b93a6f882a4 — PASS (every changed file is a content subset of the source of record, or an explicitly sanctioned allowNew file).
• Tests: 259 passed / 5 skipped, exit 0 (narrowest relevant suites: Task.spec.ts, writeToFileTool.spec.ts, writeToFileTool-partial-state-cleanup.spec.ts, removeClineFromStack-delegation.spec.ts, presentAssistantMessage-custom-tool.spec.ts).
• Changed-line coverage: 14 covered / 0 uncovered — PASS.
• ESLint --prune-suppressions --max-warnings=0: clean on every touched file; suppression counts unchanged.
• No .changeset file, no CHANGELOG edit (AGENTS.md).

Deviations recorded

• Owns the test reports a filesystem error only once across the streaming and execute phases, originally attributed to U5 (see U5).

Reproduce

git fetch https://github.com/easonLiangWorldedtech/Zoo-Code p1066/u6-execute-error-path-cleanup
node zdt.mjs split verify --contract U6.json --worktree <wt> --head 9b93a6f882a4
node zdt.mjs split measure --worktree <wt> --base e3c10401f760 --head 9b93a6f882a4
pnpm --dir src exec eslint --prune-suppressions --max-warnings=0 <touched file>