fix: improve perf by switching parentUIDToChildren to map of sets (#26863)

[jcogilvie]

Fixes #26863.

Checklist:

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[bunnyshell]

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.03%. Comparing base (63a009e) to head (71ecf42).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gitops-engine/pkg/cache/cluster.go	83.33%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #26864      +/-   ##
==========================================
- Coverage   63.04%   63.03%   -0.01%     
==========================================
  Files         414      414              
  Lines       56286    56284       -2     
==========================================
- Hits        35484    35481       -3     
- Misses      17427    17430       +3     
+ Partials     3375     3373       -2     

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[jcogilvie]

Benchmark Results

Add/Remove Operations (constant time regardless of children count)

Children per Parent	Time (ns/op)	Allocations
10	66.54	0
50	66.69	0
100	70.88	0
500	66.46	0
1000	78.54	0

Comparison with Previous Implementation

Children	Old (slice, O(k))	New (set, O(1))	Speedup
10	~115 ns	67 ns	1.7×
50	~329 ns	67 ns	5×
100	~581 ns	71 ns	8×
500	~2,600 ns	66 ns	39×
1000	~5,100 ns	79 ns	65×

Memory Impact

Children per parent	Slice	Set	Overhead
10	664B	848B	+28%
100	6.4KB	8KB	+25%

Cluster-level estimate (1,000 parent resources, average 10 children each = 10,000 parent-child relationships):

• Slice-based: ~66KB
• Set-based: ~85KB
• Additional overhead: ~18KB