feat(ldai): stamp modelKey and modelVersion on AI usage events (AIC-2850)

[atornsii]

Summary

• Read modelKey and modelVersion from the AI Config variation payload (model.modelKey / model.modelVersion) and expose them on Config via ModelKey() / ModelVersion() accessors and builder methods
• Stamp modelKey (when present) and modelVersion on all Tracker metric event payloads, alongside existing modelName/providerName fields
• Default modelVersion to 1 when absent, matching variation version handling; exclude both fields from the resumption token
• Additive/backward compatible — older payloads without the new fields continue to work

Part of AIC-2850 / AIC-2849. Depends on backend payload work (AIC-2876) shipping modelKey/modelVersion on variations.

Test plan

• go build ./... in ldai/
• go vet ./... in ldai/
• go test ./... in ldai/ (all passed)
• Verify against a staging environment once AIC-2876 payload is available

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

• AIC-2850
• Parent: AIC-2849
• Depends on: AIC-2876

Describe the solution you've provided

The Go AI SDK now parses modelKey and modelVersion from the variation's model object, exposes them through Config accessors, and includes them in the trackData stamped on every metric event. modelVersion is always emitted (defaulting to 1); modelKey is omitted when empty. Neither field is included in the resumption token.

Describe alternatives you've considered

Threading modelKey/modelVersion as explicit constructor parameters to newTracker (as in the Python SDK PR) was considered but rejected in favor of sourcing from Config, matching the existing modelName/providerName pattern.

Additional context

Mirrors python-server-sdk-ai#208.

  

[jsonbailey]

Holding based on our conversation today and we will follow up on slack before reviewing.

[jsonbailey]

We are also in the process of migrating this to the go-server-sdk-ai repository.