fix(applicationset): prevent prune state leakage in RollingSync (fixes #27261)

[sahanajc4]

## Problem

When using ApplicationSet with RollingSync strategy, changing `syncPolicy.automated.prune` from `true` to `false` does not take effect.

Subsequent sync operations continue to prune resources because the previous value (`prune: true`) persists in `status.operationState.operation.sync.prune`.

## Root Cause

- `SyncOperation.Prune` is defined as `bool`, which cannot distinguish between:
  - unset (field omitted)
  - explicitly set to false

- During RollingSync, if `prune` is not explicitly set in a new operation, the previous value from `operationState` may be reused.

## Solution

This PR fixes the issue by:

- Changing `SyncOperation.Prune` from `bool` to `*bool` to distinguish unset vs explicitly false
- Explicitly setting `prune` in every RollingSync-triggered sync operation:
  ```go
  Prune: &prune

• Safely converting *bool to bool when passing to the sync engine to maintain compatibility:

  prune := false
  if syncOp.Prune != nil {
      prune = *syncOp.Prune
  }

• Updating unit tests to use pointer semantics

• Adding a test case covering prune toggle (true → false)

Result

• prune=false is now correctly respected in subsequent RollingSync operations
• Prevents unintended resource pruning after configuration changes
• No changes to existing sync behavior beyond fixing the bug

Scope

• No changes to reconciliation logic
• No changes to gitops-engine APIs
• Fix is limited to correct propagation of prune values

Testing

• Updated existing unit tests for pointer semantics
• Added test case: "Prune toggles from true to false"

Checklist

• This is a bug fix
• The title of the PR states what changed and the related issue number
• The title of the PR conforms to the project conventions
• I've included "Fixes ApplicationSet RollingSync cannot set operation.sync.prune from true to false #27261" in the description
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them
• Does this PR require documentation updates?
• I've updated documentation as required by this PR
• I have signed off all my commits as required by DCO
• I have written unit tests for my change
• My build is green (troubleshooting builds)
• My new feature complies with the feature status guidelines
• I have added a description of what this PR solves
• Optional. My organization is added to USERS.md
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into

Fixes #27261
Signed-off-by: sahanjc4 < sahanajavgal7@gmail.com >

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.37%. Comparing base (0737418) to head (1700dc9).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #27284      +/-   ##
==========================================
+ Coverage   63.34%   63.37%   +0.02%     
==========================================
  Files         415      415              
  Lines       56694    56701       +7     
==========================================
+ Hits        35914    35934      +20     
+ Misses      17396    17380      -16     
- Partials     3384     3387       +3     

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