files: cover buffered stream write lifecycle edge cases

[Christof Marti (chrmarti)]

Summary

Tests-only follow-up to #337018, based on the latest main after that fix merged. Adds six deterministic file-service tests for the lifecycle and ordering pitfalls identified while reviewing the stream-write implementation's history:

• Multiple buffered chunks emitted together remain serialized, including partial provider writes; verify exact file/buffer offsets, maximum concurrent writes, final bytes, and size.
• A producer blocked by backpressure resumes across multiple chunks.
• The peeked prefix is written before the remaining streamed chunks, with continuous file offsets.
• A queued write to the same resource waits for pending work and handle closure, then succeeds after either a provider-write failure or a stream failure (two cases).
• An empty stream closes its handle without issuing provider writes.

Uses the existing in-memory filesystem provider so assertions cover real content and handle lifecycle, while deferred writes and event-loop yields exercise the relevant ordering. No production code changes.

Validation

• npm run gulp compile-client: passed with 0 errors.
• File Service + InMemory File Service unit suites: 39 passing.
• ESLint on the changed test file, git diff --check, and commit hygiene: passed.
• Mutation check: temporarily restored immediate stream end/error completion in generated output only. The new partial-write test and both queue-recovery tests failed as expected (3 failing). Restored the generated output and reran both suites: 39 passing.

Local validation performed on Windows; cross-platform validation is left to PR CI.

[Copilot]

Copilot review overview

🟢 Approval recommended

The tests accurately cover the intended lifecycle scenarios without production-code changes or unresolved issues.

Review effort: Balanced
Findings: None

What changed in this PR

Adds deterministic regression coverage for buffered file-write lifecycle and ordering edge cases.

Changes:

• Tests serialization, partial writes, backpressure, and peeked-prefix ordering.
• Tests queue recovery after write/stream failures.
• Verifies empty streams close without provider writes.

File	Description
src/​vs/​platform/​files/​test/​browser/​fileService.test.ts	Adds six buffered stream-write lifecycle tests and an in-memory buffered provider helper.

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