What this PR does / why we need it:

Multi-tenant feature views, such as a catalogue shared by many brands, benefit from Milvus
partition keys: a search filtered on the key only scans the matching partitions.

The partition key can be set per feature view with a tag, or for the whole store:

• Feature view tag milvus.partition_key: <field>.
• Store config partition_key: <field>, applied to every feature view that has that field.

The tag takes precedence. The field must be stored as VARCHAR or INT64; other types raise a clear
error. The setting only applies when Feast creates a collection: existing collections are left
alone, and Feast logs a warning explaining how to recreate them.

online_store:
  type: milvus
  partition_key: "brand_id"

FeatureView(
    name="products",
    ...,
    tags={"milvus.partition_key": "brand_id"},
)

Which issue(s) this PR fixes:

N/A

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

• Manual tests

• Testing is not required for this change

• Unit (Milvus Lite): the tag and the store config both mark the field as the partition key, and a
  filtered search returns only that tenant's rows; the store config is ignored for feature views
  without the field; the tag takes precedence over the config.

• Unit (mocked client): a missing field and a FLOAT field are rejected; existing collections log a
  warning.

• Server: the partition key is recorded in the collection schema and filtered search works.

Unit tests run on Milvus Lite 3.2.1 (pymilvus 3.0.2). Server tests in sdk/python/tests/integration/online_store/test_milvus_remote.py are marked integration and skip unless ZILLIZ_URI and ZILLIZ_TOKEN are set; they passed against a local Milvus 2.6.0, a local Milvus 3.0.2 server and against Zilliz Cloud. The existing Milvus unit and universal integration tests pass unchanged.

Misc

Part of a series of Milvus online store improvements for Zilliz Cloud and production Milvus, following #6882, #6895, #6917 and #6921.