refactor(deployments): replace snapshot_id/reference_id with source_ref-correlated tool refs

Introduce WatsonxToolRefBinding to correlate source_ref (flow version id)
with provider tool_id across all operation types. This replaces the prior
reference_id and snapshot_id fields with a unified structure that carries
provenance through create, bind, unbind, and remove_tool operations.
Key changes:
- Flatten API operation payloads: hoist flow_version_id onto operations,
  remove nested WatsonxApiUpdateToolReference wrapper
- Replace tools.existing_ids with inline tool_id_with_ref on bind operations
- Rename WatsonxCreateSnapshotBinding to WatsonxToolRefBinding (input) and
  WatsonxResultToolRefBinding (output, with created flag)
- Add created_app_ids to update results for connection tracking
- Raise HTTPException on contract violations in _to_api_tool_app_bindings
  instead of silently dropping unmappable bindings
- Add schema-level validation for conflicting source_ref on same tool_id
- E2E: cache tool_id→source_ref from create results, use helpers to build
  refs with distinct source_ref vs tool_id values