Honor explicit autocommit in memory file opens

[xingzhao-ji]

Summary

MemoryFileSystem._open() currently decides whether to publish writes from self._intrans, ignoring the autocommit value already resolved by AbstractFileSystem.open().

Consequently, fs.open('file', 'wb', autocommit=False) creates the target immediately and leaves its bytes visible after the queued transaction is discarded. Conversely, an explicit autocommit=True inside a transaction does not publish a new file, even though that handle is excluded from the transaction's commit queue.

Use the resolved flag for write/create/append publication and for copying an existing append target. Defaults remain determined by AbstractFileSystem.open(). Add regressions for new and existing files, commit/discard, and both explicit overrides, plus a changelog entry.

Validation

• All 30 new regression cases fail with unchanged production source.
• Memory/local/ZIP backend suites: 306 passed, 20 skipped on Linux with both Python 3.12.14 and 3.13.5.
• Independent review ran the 30 regressions and 32 additional text-mode/store variants successfully.
• Ruff 0.11.12 check and format checks, and git diff --check, passed.

The dependency-heavy full suite and pinned Ruff environment were not run. The change is limited to write/create/append handling; read/update mode handling is unchanged.

Prepared and tested with OpenAI Codex assistance. No human-review certification is claimed.