fix(sftp): close persistent SFTP channel on page dispose and guard async race

[GT-610]

• Close _status.client in dispose() to prevent channel leak when navigating away from the SFTP browser page.
• Replace ??= with explicit null check to avoid concurrent double-open race when multiple _listDir calls overlap.
• Add catch block to close and null-out stale client on non-SftpStatusError, allowing recovery on next call.

Summary by CodeRabbit

• Bug Fixes
  • Improved SFTP connection cleanup to prevent stale connections from persisting.
  • Coordinated concurrent connection attempts to avoid race conditions when opening sessions.
  • Ensured ongoing connection attempts are cleared and active connections are closed during shutdown.
  • Non-standard errors now trigger a proactive connection reset to avoid leaving a broken session open.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 53001e95-3b1a-49b9-ad25-f675c8073239

📥 Commits

Reviewing files that changed from the base of the PR and between 1b0a73d and 9efc948.

📒 Files selected for processing (1)

• lib/view/page/storage/sftp.dart

🚧 Files skipped from review as they are similar to previous changes (1)

• lib/view/page/storage/sftp.dart

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📝 Walkthrough

Walkthrough

SftpPage adds an _openingClientFuture to coordinate in-flight SFTP session opens. _listDirWithFallback now starts or awaits that future when no client exists, assigns _status.client from it, then clears the future before listing. On non-SftpStatusError exceptions the code closes and nulls _status.client, clears _openingClientFuture, and rethrows. dispose() closes any active client, clears _openingClientFuture, disposes controllers, and calls super.dispose().

Possibly related PRs

• lollipopkit/flutter_server_box#1155: Both PRs modify lib/view/page/storage/sftp.dart’s session open/listdir flow and client handling.

Suggested reviewers

• lollipopkit

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main changes: closing SFTP channel on dispose and handling async race conditions in the session opening logic.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch sftp-continue-fix

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@lib/view/page/storage/sftp.dart`:
- Around line 1038-1043: The code can race when initializing _status.client in
_listDirWithFallback: multiple callers may see _status.client == null and each
call _withSftpOpTimeout(_client.sftp()), creating duplicate sessions; introduce
a single-flight guard (e.g. a private Future<SftpClient>? _openingClientFuture)
that is checked/awaited before starting a new open, assign the resulting client
to _status.client once completed, and clear/reset the guard on success or error;
update the existing initialization logic that calls _withSftpOpTimeout('open
browser session', _client.sftp()) to use this shared in-flight future so only
one _client.sftp() is invoked concurrently.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: da3d3c1d-d977-4cd0-b842-9a2111f4f064

📥 Commits

Reviewing files that changed from the base of the PR and between 2df593c and 1b0a73d.

📒 Files selected for processing (1)

• lib/view/page/storage/sftp.dart