[kbn-evals] Code quality and dead-code cleanup

[milistu]

Summary

Structural cleanups with no behaviour change.

1. URL constants and internal header builders are exported from the public API but shouldn't be. SimpleEvaluator and KibanaScoresClient are used externally but missing from __all__. (__init__.py, api/__init__.py)
2. Seven ES-export logging helpers in utils/logging.py have zero call sites in the codebase. Remove them.
3. KibanaDatasetsClient, KibanaScoresClient, and KibanaEvaluatorsClient each have an identical __init__ (kibana_url, api_key, timeout). Extract a shared base or dataclass.
4. The connector-ID validation block is copy-pasted in correctness/evaluator.py and groundedness/evaluator.py. Extract a shared helper.
5. runner/cli/commands/run.py repeats env-var name strings that are already literals in config.py. Define them as constants in config.py and import them. The _apply_overrides() function is a slower env.update(overrides). Remove it.
6. Dead EvaluationResult.reasoning and .details fields (never set or read).
7. SimpleEvaluator uses @dataclass but its __init__ is overridden immediately, leaving an unintended __repr__ and __eq__. Replace with a plain class.
8. Five always-populated fields on EvaluationRun are typed Optional for no reason. Make them required.
9. log_evaluation_start logs the same info that the following per-evaluator lines already cover. Remove it.
10. No tests for runner/suites.py, cli/commands/run.py, or evaluators/criteria/*. No conftest.py. _RecordingAsyncClient is copy-pasted into four separate test files. Move the best version to a shared fixture.

Done when

• All items above are addressed in one PR
• No behaviour change
• Existing tests pass