What this PR does / why we need it:

RemoteOnlineStore._construct_online_read_api_json_request built the request body from row.join_keys[0] and row.entity_values[0] only. For a feature view with a composite entity key, every join key after the first was dropped before the request left the client. The feature server then joined on a partial key, matched a row, and returned it with status PRESENT. No exception was raised anywhere in the path, so callers silently received features belonging to the wrong entity.

Reproduced on 0.64.0 with a two-entity feature view keyed on (driver_id, customer_id). The request that reached the server was:

{'features': ['driver_customer_stats:conv_rate'],
 'entities': {'driver_id': [1001, 1002]}}

customer_id is not null or truncated. It is absent.

Two notes for reviewers:

1. The issue title attributes this to get_online_features, but that method already builds a correct columnar request across all keys. The defect is one layer down, in the online_read path taken when a local FeatureStore is configured with an online store of type: remote.

2. get_online_features is covered by test_columnar_entity_rows. The online_read request builder had no test coverage at all, which is why the divergence between the two went unnoticed.

This PR builds the request from every (join_key, entity_value) pair, keyed on join key name rather than position, and raises on a length mismatch between join_keys and entity_values rather than silently truncating.

Which issue(s) this PR fixes:

Fixes #6488

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests