fix: Use a shared entity_rows name as both request data and join key - #6794

What this PR does

Closes #6790.

_prepare_entities_to_read_from_online_store classified each entity_rows key with a mutually-exclusive if/elif chain:

if join_key_or_entity_name in needed_request_data:
    request_data_features[join_key_or_entity_name] = values
elif join_key_or_entity_name in join_keys_set:
    ...

A name present in needed_request_data was therefore only ever treated as request data. When an OnDemandFeatureView's RequestSource declares a field whose name matches a FeatureView's join key, the join-key branch is unreachable, join_key_values never receives the value, and every FeatureView expecting that join key fails with a missing-join-key error — even though the value was present in entity_rows all along.

get_historical_features accepts the equivalent entity_df, so the same input succeeds offline and fails online.

This PR classifies the two roles independently.

A second issue this surfaces

Making that name reachable in both mappings exposes a latent failure one line later:

data=dict(**join_key_values, **request_data_features)

dict(**a, **b) raises TypeError: got multiple values for keyword argument when the same key is in both mappings — exactly the case this change makes possible. Switched to the dict literal {**a, **b}, which permits the duplicate. Both mappings hold the same values object for a shared name, so the merge is unambiguous. test_shared_name_populates_response_once guards against the name being emitted twice.

Tests

Adds sdk/python/tests/unit/test_shared_request_data_join_key.py (5 cases), following the existing test_feature_resolution_cache.py pattern of patching _get_online_request_context:

• the shared name resolves as a join key
• the shared name appears in the result row names
• the shared name is emitted exactly once in the response
• a request-data-only name does not leak into join_key_values (control)
• an ordinary join key is unchanged (control)

Two of these fail on master (assert 'user_id' in {} — join_key_values is empty) and all five pass with this change.

Verification

Full sdk/python/tests/unit run, same environment, before and after:

The failure sets are identical in both directions — the 94 pre-existing failures are unrelated collection/dependency errors in my local environment (missing optional extras such as httpx2, boto3, pymilvus), and the +5 is exactly the new tests.

ruff check, ruff format --check, and mypy feast/utils.py are all clean on the changed files.