fix(ssh): handle empty recovery roots

[njzjz-bot]

Summary

• remove an empty previous submission-hash directory as a disposable placeholder instead of moving it into the new hash;
• preserve non-empty recovery data with the standard non-overwriting SFTP rename operation;
• tolerate only a missing source during concurrent recovery while surfacing unrelated SFTP failures and leaving an existing destination untouched;
• add mocked recovery tests that run without an SSH server.

Validation

• python -m coverage run -p --source=./dpdispatcher -m unittest -v — 168 passed, 42 skipped
• uvx pre-commit run --all-files
• uvx --from ty==0.0.17 --with '.[cloudserver,gui]' --with tomli ty check dpdispatcher/contexts/ssh_context.py tests/test_ssh_context.py
• python -m dpdispatcher -h, dpdisp --help, and dpdisp run examples/dpdisp_run.py
• make SPHINXOPTS='-D intersphinx_timeout=2' html — succeeds with existing documentation warnings

The repository-wide ty check also reports four existing allow_ref signature diagnostics in dpdispatcher/run.py and dpdispatcher/entrypoints/submit.py; neither file is changed here.

Closes #262

Coding agent: Codex
Codex version: codex-cli 0.149.0
Model: gpt-5.6-sol
Reasoning effort: xhigh

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Run ID: 2ba5702c-e3ee-4405-b026-a944c081106d

📥 Commits

Reviewing files that changed from the base of the PR and between 5d1536c and 08980fa.

📒 Files selected for processing (2)

• dpdispatcher/contexts/ssh_context.py
• tests/test_ssh_context.py

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[codecov]

Codecov Report

❌ Patch coverage is 75.75758% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.87%. Comparing base (5d1536c) to head (08980fa).

Files with missing lines	Patch %	Lines
dpdispatcher/contexts/ssh_context.py	75.75%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #640      +/-   ##
==========================================
+ Coverage   57.51%   57.87%   +0.35%     
==========================================
  Files          40       40              
  Lines        4256     4285      +29     
==========================================
+ Hits         2448     2480      +32     
+ Misses       1808     1805       -3     

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[njzjz-bot]

No blocking findings after reviewing the full diff, related code, and CI checks. Intended decision: APPROVE. GitHub does not permit njzjz-bot to approve a pull request authored by the same account, so this formal review is submitted as COMMENT.

Coding agent: Codex
Codex version: codex-cli 0.151.0
Model: gpt-5.6-sol
Reasoning effort: xhigh

[njzjz-bot]

Reviewed the complete diff and relevant surrounding code. No blocking findings. GitHub does not permit the njzjz-bot account to approve a pull request authored by njzjz-bot, so this review is submitted with the COMMENT event only.

Coding agent: Codex
Codex version: codex-cli 0.151.0
Model: gpt-5.6-sol
Reasoning effort: xhigh

[njzjz-bot]

Independent review result: no blocking issues found. GitHub prevents njzjz-bot from approving a pull request authored by the same account, so this formal review is submitted as COMMENT rather than APPROVE.

Coding agent: Codex
Codex version: codex-cli 0.151.0
Model: gpt-5.6-sol
Reasoning effort: xhigh