feat: add hooks support

[keelerm84]

Implements the Hooks specification (evaluation series + track series) to
give users and LaunchDarkly integration packages an extension point for
observing flag evaluations and track calls.

• Public API under LaunchDarkly\Hooks: Hook (abstract base class),
  Metadata, EvaluationSeriesContext, TrackSeriesContext.
• LDClient accepts a hooks option at construction and exposes
  addHook() for runtime registration.
• beforeEvaluation / afterEvaluation fire around every variation,
  variationDetail, and migrationVariation call, with afterEvaluation
  running in reverse registration order per spec 1.3.4.
• afterTrack fires after a custom event is enqueued. It does not fire
  for trackMigrationOperation (not a custom event) or for track calls
  rejected due to an invalid context.
• Hook exceptions are caught and logged at the error level; when a stage
  errors, subsequent stages receive the previous successful stage's data
  (spec 1.3.7.1).
• environmentId is intentionally omitted from the series contexts until
  the SDK can populate it reliably; the spec lists it as optional.
• Identify series (1.4) and configuration handlers (1.5) are skipped
  because they are client-side only.
• Contract test service declares evaluation-hooks and track-hooks
  capabilities and wires harness-provided hook configs through a new
  PostingHook implementation.

---

Note

Medium Risk
Adds synchronous hook execution around flag evaluation and custom event tracking, which can affect evaluation latency/ordering and introduces new error-isolation/logging paths in core client flows. Risk is mitigated by no-op defaults, hasHooks() fast-path, and tests covering ordering and exception handling.

Overview
Adds a new public hooks API (LaunchDarkly\Hooks\Hook, Metadata, EvaluationSeriesContext, TrackSeriesContext) and wires hook execution into LDClient.

LDClient now accepts a hooks constructor option and a runtime addHook() method; it runs beforeEvaluation/afterEvaluation around variation, variationDetail, and migrationVariation (with reverse order for afterEvaluation), and runs afterTrack after successful track() calls. Hook stage failures are caught/logged and isolated via the new internal Impl\Hooks\HookRunner, and evaluation error handling is widened to catch \Throwable.

Updates the contract test service to advertise evaluation-hooks/track-hooks and to instantiate harness-configured hooks via a new PostingHook, and adds unit tests for ordering, data propagation, and exception isolation. Also updates the Psalm baseline for the new code and version.

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