[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. decisions[].campaign_id and impression events[].entity_id fall back to experiment_id when invalid; decisions[].variation_id becomes JSON null. The path never logs, errors, or blocks event dispatch.

Changes

• Added pkg/event/event_id_normalizer.go with three pure helpers (IsNumericIDString, NormalizeCampaignID, NormalizeVariationID returning *string).
• Wired the normalizer into the single impression path in pkg/event/factory.go — createImpressionEvent normalizes from experiment.LayerID (falling back to experiment.ID for holdouts, which have 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 non-numeric upstream values marshal to JSON null, matching the cross-SDK contract and the existing nullable-field convention used by DecisionMetadata.CmabUUID.
• Audited existing event-builder tests per FR-011 — replaced non-numeric placeholders (holdout_123, holdout_var_123) with numeric IDs, updated holdout assertions to the normalized output with inline FSSDK-12813 references, and added unit and integration coverage for the new helpers and the full wire payload.

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 and more permissive in values.

Jira Ticket

FSSDK-12813