fix: Fix five bugs in milvus online store

[korbonits]

What does this PR do?

Fixes five bugs in sdk/python/feast/infra/online_stores/milvus_online_store/milvus.py and adds regression tests for three of them.

1. update() overwrites the collection cache (_collections) instead of updating it

_get_or_create_collection() already updates self._collections[collection_name] as a side effect and returns that entry. The assignment in update() replaced the entire _collections dict with the raw describe_collection() dict for the last table, corrupting all subsequent cache lookups for any other collection.

Fix: drop the assignment; the side effect in _get_or_create_collection() is sufficient.

2. plan() raises NotImplementedError instead of returning []

The OnlineStore base class default is return []. Raising NotImplementedError caused feast plan to crash for any project using the Milvus store.

3. online_read() has an extra full_feature_names parameter not in the base class

The parameter was unused in the method body and violates the OnlineStore interface contract (online_read takes requested_features as the last argument).

4. retrieve_online_documents_v2() crashes with TypeError when the composite key is absent from a hit

# before — bytes.fromhex(None) raises TypeError
entity_key_bytes = bytes.fromhex(
    hit.get("entity", {}).get(composite_key_name, None)
)

# after
raw_key = hit.get("entity", {}).get(composite_key_name)
entity_key_bytes = bytes.fromhex(raw_key) if raw_key else None

5. _connect() uses print() instead of logging

Connection messages now use logger.info() so they respect the application's logging configuration instead of unconditionally writing to stdout.

Tests

Three regression tests added in tests/unit/online_store/test_online_retrieval.py:

Test	Bug covered
test_milvus_update_preserves_collection_cache	_collections cache corruption in update()
test_milvus_plan_returns_empty_list	plan() raising NotImplementedError
test_milvus_retrieve_online_documents_v2_missing_entity_key	TypeError from bytes.fromhex(None)

Full Milvus unit test results:

tests/unit/online_store/test_online_retrieval.py -k milvus
→ 8 passed, 1 skipped  (7.90s)

ruff check passes with no errors.

🤖 Generated with Claude Code

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 5 additional findings.

[korbonits]

Hmm, looks like the unit test error for python 3.12 ubuntu was:

________________ tests/unit/local_feast_tests/test_e2e_local.py ________________
[gw6] linux -- Python 3.12.13 /home/runner/work/feast/feast/.venv/bin/python3
worker 'gw6' crashed while running 'tests/unit/local_feast_tests/test_e2e_local.py::test_e2e_local'
=============================== warnings summary ==============================

Maybe re-running would resolve?

[devin-ai-integration]

Devin Review found 1 new potential issue.

View 6 additional findings in Devin Review.

[devin-ai-integration]

🟡 Falsy check on distance drops valid 0.0 distance values

At line 746, if distance is used to decide whether to wrap the distance in a ValueProto(float_val=...). When distance is 0.0 (a valid distance meaning an exact match for L2 or cosine metrics), Python evaluates 0.0 as falsy, so the code returns an empty ValueProto() instead of ValueProto(float_val=0.0). Because float_val is inside a oneof val in the proto definition (protos/feast/types/Value.proto:78), ValueProto() leaves the oneof unset (WhichOneof("val") → None), while ValueProto(float_val=0.0) correctly sets it to "float_val". This is the same bug pattern the PR fixes for raw_key at line 694. The postgres online store handles this correctly with if distance is not None: (sdk/python/feast/infra/online_stores/postgres_online_store/postgres.py:777).

(Refers to line 746)

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