fix(windows): preserve binary bytes in anchored file reads

[allenliang2022]

Thinking Path

• Hermes WebUI serves workspace files and immutable media snapshots through an anchored file-descriptor path.
• On Windows, file descriptors opened without O_BINARY use text semantics.
• A valid MP4 whose first 0x1A byte occurred at offset 568 was therefore reported and served as 568 bytes even though its real size was 5,248,045 bytes.
• The non-anchored media path already adds O_BINARY; the shared anchored-open helper did not.
• This change makes non-directory anchored reads binary-safe while preserving directory-open flags.

What Changed

• Add the platform-optional O_BINARY flag to non-directory opens in open_anchored_fd().
• Preserve the existing directory-only flag path.
• Add a Windows Ctrl-Z regression test using the real anchored snapshot read path.
• Add platform-independent flag-contract tests so Linux/macOS CI still detects removal of the Windows safety flag.

Why It Matters

Without this flag, arbitrary binary workspace files can be truncated at the first DOS Ctrl-Z byte on Windows. For video snapshots this produces invalid Range lengths and browser playback failures even though the source file and checksum are valid.

Verification

• Regression test on the unpatched current origin/master: RED, actual_size == 568 instead of 4665.
• Same test after the fix: GREEN.
• Focused and neighboring tests: 23 passed, 7 skipped.
• Full four-shard Windows run: the changed tests and shard 2 were green; shards 0/1/3 reported existing Windows-environment failures. The failure sets were identical across two independent origin/master + 1 commit branches. A clean origin/master control reproduced the failures (or classified the remaining two as one platform skip and one concurrency-only pass). No failure was unique to this branch.
• python -m py_compile api/workspace.py tests/test_media_message_snapshots.py
• git diff --check

Risks / Follow-ups

• The change is limited to read-only non-directory opens. Directory opens still use O_DIRECTORY and do not receive O_BINARY.
• O_BINARY resolves to zero on platforms that do not define it, preserving existing POSIX behavior.

[greptile-apps]

Greptile Summary

The PR makes anchored file reads binary-safe on Windows while preserving directory-open behavior.

• Adds the optional O_BINARY flag to non-directory anchored opens.
• Adds a real Ctrl-Z snapshot regression test and platform-independent flag-contract tests.

Confidence Score: 5/5

The PR appears safe to merge, with the Windows binary-read correction narrowly scoped and appropriately tested.

The changed flag selection mirrors the existing non-anchored binary-read behavior, does not combine binary and directory flags, and remains a no-op on platforms without O_BINARY.

Important Files Changed

Filename	Overview
api/workspace.py	Correctly adds O_BINARY only to non-directory opens; POSIX behavior remains unchanged because the optional flag resolves to zero.
tests/test_media_message_snapshots.py	Adds focused coverage for Windows Ctrl-Z truncation and verifies that file and directory opens retain distinct flag contracts.

_{Reviews (1): Last reviewed commit: "fix(windows): open anchored files in bin..." | Re-trigger Greptile}

[nesquena-hermes]

Shipped in experimental release exp-v0.52.234. Non-directory anchored file opens now set O_BINARY where the platform provides it, so binary workspace files (video/image/archive) no longer truncate at the first 0x1A byte on Windows — Range serving and playback work correctly. Codex SAFE + 88 focused tests + combined batch suite green. Thanks @allenliang2022!

🤖 Released by the release-manager agent.