[split-1066] U5 - fix(write-to-file): capture the streaming failure once and report it once #1935

Unit U5 of the PR #1066 split (3/6).

Split plan: #703
PR: #1930
Content source of record: tag pr1066-source = 46d1d218701f0ce2d675b1b315489bacb6b0f77d (easonLiangWorldedtech/Zoo-Code).

Unit contract

handlePartial() captures the streaming failure once and reports it once — no duplicate error bubble; the authoritative execute() error is the one surfaced.

Why this unit exists on its own

Single provider group + single gate scope (streaming failure capture and its single reporting point).

Boundary

• base: 52699c6cdc7d (previous unit head, U4)
• head: 4b23b6a270aa
• chain: U4 -> U5 -> U3
• 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 +38/-13 — content subset of source
• src/core/tools/__tests__/writeToFileTool.spec.ts +229/-3 — content subset of source
• budget: 283 a+d / 2 files — UNDER-SOFT
• mutation gate: 24 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 U5.json --worktree <wt> --head 4b23b6a270aa — PASS (every changed file is a content subset of the source of record, or an explicitly sanctioned allowNew file).
• Tests: 239 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: 13 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

• The test reports a filesystem error only once across the streaming and execute phases was re-attributed from U5 to U6: it asserts the execute() error-path restructure, so it cannot pass at U5.

Reproduce

git fetch https://github.com/easonLiangWorldedtech/Zoo-Code p1066/u5-streaming-failure-capture
node zdt.mjs split verify --contract U5.json --worktree <wt> --head 4b23b6a270aa
node zdt.mjs split measure --worktree <wt> --base 52699c6cdc7d --head 4b23b6a270aa
pnpm --dir src exec eslint --prune-suppressions --max-warnings=0 <touched file>