fix(plugins): skip python dependency install on modified_diff dry run

[EmilyRagan]

Problem

POST /openc3-api/plugins/modified_diff (called by ModifiedPluginDialog.loadDiff before a plugin upgrade) can return a 504.

The endpoint runs PluginModel.install_phase2(..., diff_only: true) inline in the request. The Python dependency block was guarded by unless validate_only rather than unless dry_run, so a diff-only run still invoked uvinstall/pipinstall. For plugins that ship a pyproject.toml or requirements.txt, the request waited on PyPI (minutes, or indefinitely on air-gapped systems) and also mutated the per-plugin venv, despite modified_diff being documented as read-only.

Change

• Guard the Python install with dry_run, matching the other side effects in install_phase2.
• Add a spec asserting no install command runs when diff_only is set.

validate_only is unchanged: it already feeds dry_run, and CLI validate still needs it as a distinct mode.

Not addressed

modified_diff can still be slow for large plugins, because it does one get_object per target file and builds a System per target. That can be a follow-up if needed.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.13%. Comparing base (e3a728f) to head (708cd01).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3963      +/-   ##
==========================================
- Coverage   80.13%   80.13%   -0.01%     
==========================================
  Files         901      901              
  Lines       68370    68365       -5     
  Branches     2699     2697       -2     
==========================================
- Hits        54789    54784       -5     
- Misses      12913    12916       +3     
+ Partials      668      665       -3     

Flag	Coverage Δ
python	80.13% <ø> (ø)
ruby-api	82.65% <ø> (+0.06%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud