[AI-FSSDK] [FSSDK-12839] Normalize impression event campaign_id, variation_id, entity_id

[jaeopt]

Summary

Impression-event payloads emitted by the Python SDK can carry empty, null, or non-numeric values for decisions[].campaign_id, decisions[].variation_id, and events[].entity_id — most visibly on holdout decisions, which legitimately lack a layerId. Downstream event processing requires these fields to be numeric strings (or explicit null for variation_id), so malformed values are silently dropped or misattributed. This change normalizes those fields at the event-builder layer so every impression event conforms to the wire contract.

Changes

• Added a is_numeric_string_id helper that recognises a non-empty ASCII decimal-digit string as the only valid form for these IDs.
• In both the legacy event builder and the modern event factory, normalize decisions[].campaign_id to fall back to the decision's experiment_id when the source value is invalid, and normalize decisions[].variation_id to None when invalid.
• Derive events[].entity_id from the same normalized value as campaign_id so the two fields are guaranteed byte-equal on every impression payload.
• Limited the change to impression events; conversion events continue to derive entity_id from the conversion event's own id.

Jira Ticket

FSSDK-12839

[jaeopt]

redundant