fix: saving workflow with same name as deleted workflow

[doc-han]

Description

This PR introduces a new way to deal with reusing workflow names.

When a name collision happens between workflows

We've updated the error message to inform users that the workflow could be pending deletion

When a workflow is deleted

We add _del<number> to reduce the chances of a collision.

Closes #4156

Validation steps

1. delete one of your workflows.
2. create a new workflow with the same name.
3. you should have no errors showing up and be able to save successfully.

Additional notes for the reviewer

1. (Is there anything else the reviewer should know or look out for?)

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

You can read more details in our Responsible AI Policy

Pre-submission checklist

• I have performed a self-review of my code.
• I have implemented and tested all related authorization policies. (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.00%. Comparing base (b264f13) to head (fbfa34d).

Files with missing lines	Patch %	Lines
lib/lightning/workflows.ex	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4165      +/-   ##
==========================================
- Coverage   89.05%   89.00%   -0.06%     
==========================================
  Files         425      425              
  Lines       19677    19687      +10     
==========================================
- Hits        17523    17522       -1     
- Misses       2154     2165      +11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[doc-han]

@taylordowns2000 this is ready

[taylordowns2000]

thank you han!