fix: Use configured timestamp column names in RemoteOnlineStore.online_write_batch

[franciscojavierarceo]

Summary

• RemoteOnlineStore.online_write_batch hardcoded "event_timestamp" and "created" as column names when building the request body for the feature server. The server-side _convert_arrow_fv_to_proto resolves columns by the feature view's configured batch_source.timestamp_field and batch_source.created_timestamp_column. When those differ from the hardcoded defaults (e.g. "created_at"), the server throws a KeyError.
• Now uses the feature view's configured column names, falling back to the previous defaults when unset.
• Adds 3 unit tests covering default columns, custom columns, and empty created_timestamp_column fallback.

Closes #6595

Test plan

• Existing 22 tests in test_remote_online_store.py still pass
• New TestRemoteOnlineStoreWriteBatch tests verify:
  • Default column names (event_timestamp, created) are used when configured
  • Custom column names (ts, created_at) appear in the request body instead of hardcoded defaults
  • Empty created_timestamp_column falls back to "created"
• Integration test with a remote feature server and a feature view using non-default timestamp columns

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[franciscojavierarceo]

@codex review please

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Guard ODFV writes before reading batch_source

When a remote FeatureStore.write_to_online_store() call targets an OnDemandFeatureView with write_to_online_store=True, PassthroughProvider.online_write_batch forwards that ODFV to this online store, and _convert_arrow_odfv_to_proto has already supplied artificial event/created timestamps. ODFVs do not define batch_source, so reading table.batch_source here raises AttributeError before the request is posted; the previous hard-coded timestamp-column path did not have this failure mode. Please fall back to the default columns when the table has no batch source.

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