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

[jaeopt]

Summary

Normalize three ID fields on outgoing decision events — decisions[].campaign_id, decisions[].variation_id, and impression events[].entity_id — so the wire payload is byte-equivalent across SDKs for every decision type (experiment, feature test, rollout, holdout). The normalization path is silent and never drops, defers, or fails event dispatch.

Per the updated FSSDK-12813 spec, campaign_id and entity_id accept any non-empty string (numeric or opaque, e.g. "default-12345", "layer_abc"); the fallback to experiment_id fires only when the value is empty string, nil, or missing. variation_id keeps the stricter numeric-string-only contract — empty/whitespace/non-numeric values normalize to nil. Non-string types are out of scope.

Changes

• Added a shared Helpers::EventIdValidator module with two predicates: non_empty_string? (used for campaign_id and entity_id, accepts any non-empty string including opaque IDs) and numeric_string? (used for variation_id, requires decimal digits only). The normalization helpers apply the spec's substitution rules: campaign_id falls back to experiment_id only when empty/nil; invalid variation_id becomes nil.
• Routed every impression decision through the new normalization helper in EventFactory#create_impression_event_visitor so the rule applies uniformly to experiment, feature test, rollout, and holdout decisions with no per-type branching.
• Guaranteed that the impression events[].entity_id reuses the same normalized campaign_id byte-for-byte (impression-only; conversion event entity_id is unchanged).
• Added unit coverage for both predicates and the normalizers (including opaque/whitespace passthrough for campaign_id and nil-fallback for variation_id) and integration coverage on the EventFactory wire payload covering nil, empty, whitespace, non-string, non-numeric-placeholder, and opaque-prefixed (e.g. default-12345) inputs, plus a holdout scenario.
• Updated a pre-existing wire-payload assertion in spec/project_spec.rb to expect variation_id: nil (was '') for the rollout-no-match path, per the variation_id normalization rule (FR-011).

Jira Ticket

FSSDK-12813