fix(plugins): protect modified files on upgrade and surface diff failures

[EmilyRagan]

Problems

1. A failed diff was reported as "no conflicts." PluginModel.modified_diff rescued every error and returned [], and ModifiedPluginDialog.loadDiff also turned any request failure (including a 504) into []. The dialog then said "None of {plugin}'s modified files conflict with the new plugin. Installing will proceed normally.", even though no check had completed.
2. The upgrade dialog was wrong without Version History. The dialog is shared by Core and Enterprise and tells users that installing replaces modified files and saves the old content to Version History. TargetModel#apply_upgrade_version only runs when the Enterprise VersionStore is present, so in Core the modified copy under targets_modified/ stays in place and keeps overriding the new plugin's file. Nothing is versioned, and the old "delete modified files" option was removed, so Core users had no way to take the plugin's version.
3. Upgrades could lose modified files in Enterprise without Version History. TargetModel#version_store_available? only checked that the Enterprise VersionStore class loads, not that it is enabled. With OPENC3_VERSION_HISTORY_DIR unset, VersionStore.commit does nothing, but apply_upgrade_version still deleted the modified copy. The user's content was gone with no saved version.

Changes

• PluginModel.modified_diff no longer swallows errors. The controller already returns them as a 500 with a message.
• ModifiedPluginDialog shows an inline error when the diff fails. It explains that continuing keeps all modified files, which then override the new plugin. The request sets Ignore-Errors for 5xx so the global banner, which would include the truncated plugin hash, doesn't also appear.
• PluginsTab passes the existing scriptVersionsEnabled flag (from /openc3-api/info) into the dialog as versionHistory. When it is false (Core, or Enterprise without OPENC3_VERSION_HISTORY_DIR), an upgrade shows the modified-file list and the DELETE MODIFIED checkbox, as it did before Version History, plus a note that kept files override the plugin. version_history_files is not sent in that case.
• version_store_available? now also requires VersionStore.enabled?. When Version History is disabled, the backend ignores version_history_files and leaves modified copies in place, even if the frontend sends that list.
• The upgrade section of the Playwright plugins test checks the delete box when it is shown.

Testing

• openc3/spec/models/plugin_model_spec.rb: 61 examples, 0 failures
• openc3/spec/models/target_model_spec.rb: 109 examples, 0 failures, including new version_store_available? cases (enabled, disabled, gem absent)
• openc3-cosmos-cmd-tlm-api/spec/controllers/plugins_controller_spec.rb: 32 examples, 0 failures, including a new 500 case
• ESLint with --max-warnings 0 on the changed Vue files, and Prettier on the Playwright spec
• Playwright not run locally

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 22.72727% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.12%. Comparing base (e3a728f) to head (254dd97).

Files with missing lines	Patch %	Lines
...nc3-vue-common/src/tools/admin/tabs/PluginsTab.vue	0.00%	12 Missing ⚠️
...ue-common/src/tools/admin/ModifiedPluginDialog.vue	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3969      +/-   ##
==========================================
- Coverage   80.13%   80.12%   -0.02%     
==========================================
  Files         901      901              
  Lines       68370    68374       +4     
  Branches     2699     2650      -49     
==========================================
- Hits        54789    54784       -5     
- Misses      12913    12924      +11     
+ Partials      668      666       -2     

Flag	Coverage Δ
frontend	67.00% <15.00%> (+0.01%)	⬆️
python	80.13% <ø> (ø)
ruby-api	82.34% <ø> (-0.25%)	⬇️
ruby-backend	85.67% <100.00%> (+0.01%)	⬆️

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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

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