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

[jaeopt]

Summary

Normalize three decision-event ID fields uniformly across every decision type (experiment, feature test, rollout, holdout) in the event-builder layer so the wire payload is byte-equivalent across SDKs for the same input. decisions[].campaign_id and impression events[].entity_id fall back to experiment_id only when the value is empty string or None — any non-empty string (numeric like "12345" or opaque like "default-12345", "layer_abc") passes through unchanged. decisions[].variation_id retains the strict numeric-string contract and falls back to None for empty/whitespace/non-numeric input. The path never logs, raises, or drops dispatch.

Changes

• Added optimizely/event/event_id_normalizer.py with pure helpers (is_non_empty_string, is_numeric_id_string, normalize_campaign_id, normalize_variation_id). normalize_campaign_id accepts any non-empty string and falls back to experiment_id only on empty/None; normalize_variation_id keeps the strict decimal-digit contract and returns None otherwise.
• Wired the normalizer into event_factory.py (current) and event_builder.py (legacy) on the single impression path so both producers emit identical output.
• Reused the normalized campaign_id for events[].entity_id so the two fields are byte-equivalent by construction. Typed Decision.variation_id as Optional[str] so invalid inputs serialize as JSON null.
• Audited existing event-builder tests per FR-011 — happy-path fixtures already use numeric IDs; integration tests updated to assert opaque-string passthrough for campaign_id / entity_id and null normalization for variation_id.

Jira Ticket

FSSDK-12813