perf: cache menu tree lookups

[ruibaby]

What type of PR is this?

/kind improvement

What this PR does / why we need it:

This PR caches menu tree data used by the theme menu finder, so repeated finder calls can reuse an already-built tree instead of listing all menu and menu item extensions every time.

It extracts the tree-building logic into MenuTreeCache, shares in-flight loads, and invalidates the cache when menu-related extension watch events arrive. It also publishes a MenuItemReconciledEvent after menu item reconciliation so status changes that affect rendered menu labels and links refresh the cached tree.

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

This PR was prepared with AI assistance and reviewed before submission.

Validation:

• ./gradlew :application:test --tests "*MenuFinderImplTest" --tests "*MenuItemReconcilerTest"
• ./gradlew :application:spotlessCheck :application:test

Does this PR introduce a user-facing change?

NONE

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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Once this PR has been reviewed and has the lgtm label, please assign johnniang for approval. For more information see the Code Review Process.

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Details

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[codecov]

Codecov Report

❌ Patch coverage is 88.05970% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.11%. Comparing base (42c374d) to head (f89133e).
⚠️ Report is 588 commits behind head on main.

Files with missing lines	Patch %	Lines
...run/halo/app/theme/finders/impl/MenuTreeCache.java	87.30%	6 Missing and 10 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #10022      +/-   ##
============================================
+ Coverage     59.55%   61.11%   +1.55%     
- Complexity     3812     4122     +310     
============================================
  Files           677      708      +31     
  Lines         23248    23855     +607     
  Branches       1500     1572      +72     
============================================
+ Hits          13846    14579     +733     
+ Misses         8764     8520     -244     
- Partials        638      756     +118     

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