[AI-FSSDK] [FSSDK-12813] Normalize decision event campaign_id, variation_id, and entity_id

[jaeopt]

Summary

Normalize the campaign_id, variation_id, and entity_id fields on outgoing decision events uniformly across every decision type (experiment, feature test, rollout, holdout) so the SDK emits cross-SDK byte-equivalent wire output. Per the updated FSSDK-12813 spec: decisions[].campaign_id and impression events[].entity_id require a non-empty string (any character content — IDs may be opaque, e.g. "default-12345", "layer_abc") and fall back to experiment_id only when empty; decisions[].variation_id retains a strict numeric-string contract and becomes JSON null for empty / non-numeric values. The path never logs, errors, or blocks event dispatch.

Changes

• Added pkg/event/event_id_normalizer.go with pure helpers:
  • IsNonEmptyString — relaxed predicate for campaign_id / entity_id.
  • IsNumericIDString — strict predicate, still used for variation_id.
  • NormalizeCampaignID(campaignID, experimentID) — returns campaignID when non-empty (opaque values allowed); otherwise falls back to experimentID.
  • NormalizeVariationID(variationID) *string — returns a pointer when variationID is a numeric string; otherwise nil so the field serializes as JSON null.
• Wired the normalizer into the single impression path in pkg/event/factory.go — createImpressionEvent normalizes from experiment.LayerID (falling back to experiment.ID only when LayerID is empty, e.g. holdouts with no layer) and reuses the same value for EntityID. createImpressionVisitor re-applies normalization as defense-in-depth so the two fields cannot drift. Conversion path is untouched.
• Changed Decision.VariationID to *string so empty / non-numeric upstream values marshal to JSON null, matching the cross-SDK contract and the existing nullable-field convention used by DecisionMetadata.CmabUUID.
• Updated event-builder tests for the relaxed campaign_id / entity_id contract: added TestIsNonEmptyString, TestNormalizeCampaignID_PassesThroughOpaqueCampaignID, TestNormalizeCampaignID_PassesThroughHoldoutPlaceholder, TestNormalizeCampaignID_PassesThroughWhitespaceCampaign, and TestImpressionEvent_PassesThroughOpaqueLayerID to assert that non-numeric / opaque values now pass through unchanged. Empty-LayerID holdout fallback assertions and the strict variation_id tests are unchanged.

Notes

• Decision.VariationID changing from string to *string is a Go-source-level breaking change for any downstream consumer that constructs event.Decision literals directly (no such usages exist in this repo outside the event package itself). The on-the-wire JSON contract is unchanged in shape.
• Non-string ID types are out of scope on Go (the fields are typed string); the spec relaxation is therefore purely a predicate change.

Jira Ticket

FSSDK-12813