Improve secret variables error on software upload.

[getvictor]

#24899
Better errors when secret variables used in software scripts are missing in the database. The name field indicates which script has the issue.

{
  "message": "Validation Failed",
  "errors": [
    {
      "name": "install script",
      "reason": "Couldn't add. Secret variables \"$FLEET_SECRET_INVALID1\", \"$FLEET_SECRET_INVALID0\" missing from database"
    },
    {
      "name": "post-install script",
      "reason": "Couldn't add. Secret variable \"$FLEET_SECRET_INVALID2\" missing from database"
    },
    {
      "name": "uninstall script",
      "reason": "Couldn't add. Secret variable \"$FLEET_SECRET_INVALID3\" missing from database"
    }
  ],
  "uuid": "49d671dc-636a-4966-ba2a-c159302f0f5c"
}

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Added/updated tests
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 15 lines in your changes missing coverage. Please review.

Project coverage is 63.82%. Comparing base (5f4400b) to head (22ccf72).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
ee/server/service/software_installers.go	62.06%	11 Missing ⚠️
server/service/base_client_errors.go	85.71%	2 Missing and 1 partial ⚠️
ee/server/service/maintained_apps.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #25052   +/-   ##
=======================================
  Coverage   63.82%   63.82%           
=======================================
  Files        1613     1613           
  Lines      153431   153487   +56     
  Branches     3985     3985           
=======================================
+ Hits        97922    97967   +45     
- Misses      47716    47727   +11     
  Partials     7793     7793           

Flag	Coverage Δ
backend	64.70% <78.57%> (+<0.01%)	⬆️

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[iansltx]

LGTM!