feat: add WorkspaceReclaimPolicy to Stack spec

[rquitales]

Proposed Changes

This PR introduces the WorkspaceReclaimPolicy option in the Stack spec. By default, it is set to Retain, preserving the current behavior where a Workspace (along with its associated StatefulSet and pod) is not deleted after a successful sync. When set to Delete, the Workspace pod is removed after a successful sync, effectively scaling the workload down to zero.

Testing

• Added envtests to verify that the Workspace object is deleted.
• Manually validated on a GKE cluster.

Related Issues (Optional)

Fixes #694

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 51.38%. Comparing base (59017f2) to head (a77a9a3).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...tor/internal/controller/pulumi/stack_controller.go	33.33%	8 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #804      +/-   ##
==========================================
- Coverage   51.44%   51.38%   -0.07%     
==========================================
  Files          30       30              
  Lines        4216     4231      +15     
==========================================
+ Hits         2169     2174       +5     
- Misses       1862     1870       +8     
- Partials      185      187       +2     

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