feat: Add the override marker to the flag and segment models and mark evaluations

[kinyoklion]

Summary

The OVERRIDE specification requires an evaluation to be marked as override-affected when any definition it read carried the override marker: the evaluated flag, a prerequisite at any depth, or a segment consulted during matching, whether or not the segment matched. The marking propagates upward only: a prerequisite's own record reflects only the definitions that its own subtree read, while the evaluation that requested it accumulates the result. An evaluation that fails is still marked when it read a marked definition, and a requested-type mismatch keeps the marking on its error reason.

• DataModel.FeatureFlag and DataModel.Segment carry a transient isOverride marker. It is never serialized and never read from JSON. markedAsOverride() returns a shallow copy with the marker set, sharing nested collections and preprocessing with the source and never modifying it. The override layer will use it in the next change.
• The evaluator tracks the marking per evaluation scope. The scope starts from its own flag's marker, each segment read can set it, and the value is saved, reset to the prerequisite's own marker, and restored around every prerequisite evaluation. Each result and each prerequisite record carries the marking on its reason.
• EvalResult.isOverrideAffected() reads the reason's indicator, and withOverrideAffected(boolean) returns a copy so that the shared precomputed results are never mutated.
• The server SDK now depends on launchdarkly-java-sdk-common 2.6.0, which adds EvaluationReason.isOverrideAffected() and the overrideAffected JSON property (written only when true). CI for this change cannot pass until that version is released.

Flag overrides are currently experimental and subject to change.

This PR is based on the file data reliability branch (rlamb/overrides-java-filedata-reloader) so the series applies in order; retarget it to feat/overrides once that branch merges.

The existing file data source keeps its current behavior; this change does not touch its code paths.

SDK-3246

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Note

Overview
Adds experimental flag-override tracking so evaluations can be labeled when they used override-store flag or segment definitions (not LaunchDarkly-served data).

Data model: FeatureFlag and Segment get a transient isOverride flag (never serialized) and markedAsOverride() shallow copies for the future override layer.

Evaluation: The evaluator accumulates override impact per scope—the evaluated flag, any prerequisite subtree, and any segment consulted during matching (match or not)—and sets EvaluationReason.overrideAffected on the final result and prerequisite records. Marking flows up only; each prerequisite record reflects its own subtree. EvalResult exposes isOverrideAffected() / withOverrideAffected() without mutating shared precomputed results. InputValidatingEvaluator keeps the marker on type-mismatch and malformed-flag errors.

Dependency: Bumps launchdarkly-java-sdk-common to 2.6.0 for EvaluationReason.isOverrideAffected() / JSON support.

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[kinyoklion]

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