feat(multiprovider): aggregate child provider events, state, and tracking

[jonathannorris]

Summary

• MultiProvider now observes child provider events and aggregates them into a single provider state using worst-wins precedence: FATAL > NOT_READY > ERROR > STALE > READY.
• PROVIDER_CONFIGURATION_CHANGED from any child is forwarded to the SDK.
• Implements track, forwarding to every child provider that isn't NOT_READY or FATAL; a throwing provider is logged and doesn't stop the rest.
• Duplicate provider names are disambiguated (name, name-1, name-2) instead of silently overwriting each other in the provider map.

Implementation

EventProvider gains addEventObserver/removeEventObserver so a composite provider can observe its children without going through the API-level event bus. Observers are registered during initialize and removed during shutdown.

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
• feat(multiprovider): aggregate child provider events, state, and tracking #2004 child provider event aggregation, state, and tracking (this PR)
• 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 81.11111% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.02%. Comparing base (a8e6644) to head (d1bbc8e).

Files with missing lines	Patch %	Lines
...v/openfeature/sdk/multiprovider/MultiProvider.java	81.81%	13 Missing and 17 partials ⚠️
...c/main/java/dev/openfeature/sdk/EventProvider.java	73.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2004      +/-   ##
============================================
- Coverage     92.52%   92.02%   -0.50%     
- Complexity      728      781      +53     
============================================
  Files            60       60              
  Lines          1739     1907     +168     
  Branches        202      223      +21     
============================================
+ Hits           1609     1755     +146     
- Misses           80       87       +7     
- Partials         50       65      +15     

Flag	Coverage Δ
unittests	92.02% <81.11%> (-0.50%)	⬇️

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