Let a MountCore handle go when its landing at release fails

[bytecii]

Summary

MountCore.release flushed the handle and then popped it, on both hosts. If the flush raised, the pop never ran. That happens when a pwrite is refused by a policy or a mount mode (opening an existing file for write without O_TRUNC checks neither), or when the backend fails. The handle and its buffered bytes then stayed in the table until the core went away, because no adapter retries a release:

• FUSE: the kernel ignores the error a RELEASE returns.
• SFTP: asyncssh and the TS server forget the SFTP handle before calling release, and sshfs never waits for the FXP_CLOSE reply.
• codex-exec: releases once, in a finally.

release now pops the handle in a finally and still raises the failure to the adapter. This follows Linux close(2), which frees the descriptor even when it reports an error.

Found while working on #1636. Its MountCore.sync runs at the end of every sandbox line, and it would otherwise have retried such a leaked handle and logged a warning after every line. This change is independent of #1636 and is based on main.

Test plan

• New tests, Python tests/mount/test_core.py and TS core.test.ts: on a read-only workspace, a release whose landing is refused raises, the handle is gone, and the file still holds its old bytes. Both tests fail on main.
• pytest for mount/fuse/server/runtime; vitest for node mount/fuse and server ssh; tsc for node.
• Symbol and layout parity (strict), pre-commit, mypy.

🤖 Generated with Claude Code

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; failed releases free their handles while preserving error reporting.

Summary

MountCore.release now removes the handle in finally in both languages, while still passing any write failure to the caller.

• Mount releases drop handles even when buffered writes fail.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A["release(handle)"] --> B["Attempt flush"]
  B -->|Success| C["finally: remove handle"]
  B -->|Failure| C
  C --> D{"Did flush fail?"}
  D -->|No| E["Return"]
  D -->|Yes| F["Pass error to caller"]

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