feat!: Add per-execution runId, at-most-once tracking, and cross-process tracker resumption

[jsonbailey]

Summary

• Per-execution runId: Every tracker gets a unique runId (UUID) included in all track event payloads, enabling per-execution billing isolation
• At-most-once tracking: Each metric type (duration, tokens, feedback, success/error, time-to-first-token) can only be tracked once per tracker instance — duplicates are dropped with a log warning
• CreateTracker() factory on ILdAiConfigTracker: Each call returns a new tracker with a fresh runId and independent tracking state. Returns null when the config is disabled
• ResumptionToken property on ILdAiConfigTracker: URL-safe Base64-encoded JSON containing { runId, configKey, variationKey, version } for cross-process tracker reconstruction
• CreateTracker(string token, Context context) on ILdAiClient: Reconstructs a tracker from a resumption token with the original runId, enabling deferred feedback from a different process. Model and provider names are set to empty strings on reconstruction

Test plan

• CreateTrackerReturnsNullWhenDisabled — factory returns null for disabled configs
• CreateTrackerReturnsNewTrackerWhenEnabled — factory returns distinct instance with same config
• CreateTrackerReturnsTrackerWithFreshRunId — each tracker gets a unique runId
• CreateTrackerReturnsTrackerWithIndependentTrackingState — at-most-once state is independent per tracker
• CreateTrackerCanBeCalledMultipleTimes — factory produces multiple independent trackers
• ResumptionTokenContainsExpectedFields — token contains exactly runId, configKey, variationKey, version
• ResumptionTokenIsUrlSafeBase64 — no +, /, or = characters
• ResumptionTokenIsConsistentAcrossCalls — same token on repeated access
• CreateTrackerFromResumptionTokenRoundTrips — full round-trip preserves runId
• CreateTrackerFromResumptionTokenSetsEmptyModelAndProvider — reconstructed tracker has empty model/provider
• CreateTrackerFromInvalidTokenThrows — malformed input throws ArgumentException
• CreateTrackerFromNullTokenThrows — null input throws ArgumentNullException
• CreateTrackerFromTokenMissingRunIdThrows — incomplete payload throws ArgumentException
• All 72 tests pass (64 existing + 8 new)

🤖 Generated with Claude Code