feat: add contract tests for file-based flag overrides

[kinyoklion]

Adds a contract test suite for the SDK flag-overrides feature (OVERRIDE spec), gated on a new flag-overrides capability so SDKs that don't advertise it skip everything.

Configuration follows the shared-filesystem arrangement already used for the TLS custom CA file: the harness writes override documents to temp files and passes absolute paths in a new overrides config block (filePaths, duplicateKeysHandling, watch, poll, pollIntervalMs). An OverrideFile helper creates, rewrites (deliberately non-atomic truncate-and-write, since SDKs must tolerate mid-write reads), and clears the files during a test.

Coverage:

• Static: flagValues precedence with the OFF reason and isOverride: true; a full flag override evaluating targeting rules (RULE_MATCH); passthrough for non-overridden flags with isOverride strictly absent; an overridden flag rule resolving an overridden segment through the merged view; all-flags state reflecting overrides.
• Uninitialized client (blocking mock stream + initCanFail): the overridden flag serves its override value and reason; a non-overridden flag still returns the default with CLIENT_NOT_READY.
• Summary events: an override counter carries "override": true (new overrideFlagCounter matcher) while an ordinary flag's counter carries no marker (proven by the existing exact-key matcher), and a tracked (trackEvents: true) override evaluation produces no individual feature event — the payload's exact item set is index + summary only.
• Multi-file merging with ignore duplicate handling (first file wins, later files still merged) and YAML documents.
• Hot reload, parameterized over watch mode and poll mode: add, change, and remove overrides on a running client, plus malformed-file handling (the last good overrides stay in effect across a bad edit, then recovery after the fix).

One note on reason assertions: the typed ldreason.EvaluationReason in the response model drops JSON properties it doesn't know, which would silently discard isOverride. The suite therefore asserts reasons at the raw-JSON level via a small same-package helper, which also lets the passthrough test enforce the spec's "omitted when false" serialization rule strictly.

Verified end-to-end against the Go SDK's in-progress implementation: the new suite passes in both reload modes, and the full harness run (4945 tests) stays green.

SDK-2657