feat: add support for custom judges via evaluation metric key

[knfreemLD]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

https://launchdarkly.atlassian.net/browse/REL-11511
See tech spec at https://docs.google.com/document/d/1lzYwQqCcTzN_2zkxJZDfJtgUcEJ4jbpx0KSsJ2bRENw/edit?tab=t.0#heading=h.69bdm7karsxh

Describe the solution you've provided

Updating the SDK to check the AI Config's evaluationMetricKey property which now exists. Also added missing tests from previous implementation, and fallback to the original evaluationMetricKeys list.

Describe alternatives you've considered

Provide a clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context about the pull request here.

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Note

Introduces single-key judge evaluation with backward compatibility and test coverage.

• Judge now uses evaluationMetricKey (fallback to first in evaluationMetricKeys) and validates responses accordingly
• EvaluationSchemaBuilder builds schema for a single metric key and can return None when absent
• Models updated: AIJudgeConfig(Default) include evaluation_metric_key and serialize evaluationMetricKey; evaluation_metric_keys retained only for backward compatibility
• LDAIClient.__evaluate now returns the flag variation; judge_config extracts evaluation_metric_key from that single variation to prevent race conditions
• create_judge, chat/agent paths adjusted to new types; minor cleanup of unused imports/comments
• New tests cover Judge behavior, schema builder, model serialization, and client extraction/consistency

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[jsonbailey]

Since we are sub 1.0 release as long as we can guarantee the api is always returning the new single key we should be able to just drop this and do a breaking change. They only thing that really makes this breaking is people will need to update their defaults if they defined it. If you want to drop it now update the PR to be "feat!: ".

I won't block if you want to leave this in for a little while but it likely isn't necessary. The real question is how long do we want to continue sending the old values in the API as that is what will break older SDKs.

[knfreemLD]

For now we want to make sure this is non-breaking, but soon we're going to remove "legacy" support. For keeping this change as minimal and safe as possible I'd err on the side of caution and keep it in for the time being.