feat(multiprovider): add ComparisonStrategy

[jonathannorris]

Summary

• Adds ComparisonStrategy, which evaluates every configured provider in parallel and compares the results.
• When all providers agree, the fallback provider's evaluation is returned. On mismatch, an optional callback receives the flag key and each provider's evaluation in registration order, and the fallback result is still returned.
• Provider errors, timeouts, and null evaluations are surfaced as structured ProviderError entries on the returned evaluation.

Usage

Strategy strategy = new ComparisonStrategy("primary", (flagKey, results) ->
        log.warn("providers disagree on {}: {}", flagKey, results));
FeatureProvider provider = new MultiProvider(List.of(primary, candidate), strategy);

Notes

Default timeout is 30s; a constructor overload accepts a caller-supplied ExecutorService and timeout.

Related PRs

This is one of three independent PRs that together close the multi-provider gaps identified in #1882. They branch off main separately and can be reviewed and merged in any order. Together they replace #1897.

• feat(multiprovider): add ComparisonStrategy #2003 ComparisonStrategy (this PR)
• feat(multiprovider): aggregate child provider events, state, and tracking #2004 child provider event aggregation, state, and tracking
• feat(multiprovider): execute child provider hooks during evaluation #2005 child provider hook execution

Relates to #1882

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
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

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.91%. Comparing base (a8e6644) to head (62b831a).

Files with missing lines	Patch %	Lines
...nfeature/sdk/multiprovider/ComparisonStrategy.java	89.47%	7 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2003      +/-   ##
============================================
+ Coverage     92.52%   92.91%   +0.38%     
- Complexity      728      756      +28     
============================================
  Files            60       61       +1     
  Lines          1739     1834      +95     
  Branches        202      220      +18     
============================================
+ Hits           1609     1704      +95     
+ Misses           80       79       -1     
- Partials         50       51       +1     

Flag	Coverage Δ
unittests	92.91% <89.47%> (+0.38%)	⬆️

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