fix(write-to-file): capture streaming failure once, report it once (split 3/6 of #1066)

[easonLiangWorldedtech]

U5 — streaming failure capture + single error reporting

Part of the PR Zoo-Code-Org#1066 split (tracking issue Zoo-Code-Org#703) split (content source of record: 72143527fd33306e5541116093c2cbf803cce9e0..pr-1066-audit).

Why this unit exists: handlePartial captures the streaming failure once and reports it once - no duplicate error bubble; the authoritative execute() error is the one surfaced.

Boundaries

• base: 9d78a76b8e6b
• head: feedc5dafbf4
• content source: 72143527fd33306e5541116093c2cbf803cce9e0..pr-1066-audit (local)

Fidelity (machine-verified)

zdt split verify --contract U5.json --worktree <wt> --head feedc5dafbf4

Result: PASS — standalone 283 a+d / 2 files (UNDER-SOFT)

• src/core/tools/WriteToFileTool.ts: OK (content subset of source)
• src/core/tools/__tests__/writeToFileTool.spec.ts: OK (content subset of source)

Design contract

• issue: [BUG] Agent loop stalls permanently when write_to_file partial streaming hits a filesystem error (EROFS/EACCES) Zoo-Code-Org/Zoo-Code#703

Verification (this unit, as pushed)

• Tests: 239 passed / 5 skipped
• changed-line coverage: 13 covered / 0 uncovered — PASS
• ESLint: clean on every touched file; suppression counts unchanged
• No .changeset file, no CHANGELOG edit.

Recreate policy

If the bot stalls on a pre-merge check and the existing head cannot obtain bot review/approval (empty-commit re-trigger attempted and failed), the unit is recreated from the tagged content source of record — never from a per-PR head. At most 1 PR per issue.

[easonLiangWorldedtech]

Moved to the upstream repo (base = main); the split chain is tracked in Zoo-Code-Org#703.