fix: report Agent file mutation failures

[dvd233]

Summary

file_write and file_delete currently report success even when storage responds with an HTTP error or the request fails. The Agent then receives that false success in its next model request.

Changes

• Add opt-in failure propagation to the existing storage write/delete methods and enable it for these two Agent tools. Other callers retain their current best-effort behavior.
• Preserve successful output, request data, and batching. Failed mutations are not automatically retried.
• Add 16 module regression tests and six browser tests covering HTTP failures, network failures, and success through the conversation loop.

Validation

• Before the fix: eight fault-injection assertions fail because the tools return success; eight controls pass.
• After the fix: all 127 unit tests and the default coverage check pass. Modified executable statements are fully covered; modified-line branch entries are 15/16 covered (93.75%).
• A separate whole-file coverage probe including every storage/fileTools helper falls below the global thresholds; the figures above describe the modified lines, not whole-file coverage.
• Six Chromium tests pass with synthetic API responses; the next model request receives the tool result. No live provider, account credentials, or real storage mutations are used.
• Full ESLint check, formatting, and git diff --check pass.
• Production build passes on Windows with Git Bash as the process-local script shell and pnpm build --env-mode=loose. Plain pnpm build cannot execute the existing POSIX NODE_ENV=production script in Windows cmd.exe.

Related Issues

Closes #47.

Checklist

• pnpm run lint passes (same ESLint invocation, without automatic fixes)
• pnpm build passes (Windows shell wrapper described above)
• Documentation updated if applicable (tool schemas and successful outputs are unchanged)
• Self-reviewed the code changes
• AI code review passed (auto-triggered on PR)

AI assistance was used for diagnosis, implementation, and test development.

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