What this PR does / why we need it:

GetOnlineFeaturesRequest and GetOnlineDocumentsRequest accept include_feature_view_version_metadata, and the feature server passes it on to the store. The store fills metadata.feature_view_metadata on the response proto. The REST layer then dropped the field, so over HTTP the flag had no effect.

This PR fixes all three places where the field was lost:

• feast/feature_server_utils.py: _metadata_to_dict now emits feature_view_metadata when it is non-empty. It uses the same shape as MessageToDict(preserving_proto_field_name=True), so a default name/version is omitted, as the module docstring promises. When the flag is not set, the output is unchanged.
• feast/feature_server.py: adds FeatureViewMetadataResponse and a feature_view_metadata field to OnlineFeaturesMetadataResponse, so the OpenAPI schema documents what the endpoint returns.
• feast/infra/online_stores/remote.py: _build_online_response_from_json parses feature_view_metadata back into the proto. A client that uses RemoteOnlineStore with include_feature_view_version_metadata=True already sends the flag, and now gets the metadata back.

The flag was added in #6101. The response model from #6142 already filtered the field out, and #6015 then dropped it from the custom dict builder, so it has never come back over REST.

Which issue(s) this PR fixes:

Fixes #6922

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

New tests:

• test_feature_server_utils.py::TestFeatureViewVersionMetadata: checks that convert_response_to_dict matches MessageToDict for feature_view_metadata (version 2 and version 0), and that the key is absent when the metadata was not requested.
• test_feature_server.py::test_get_online_features_returns_feature_view_version_metadata: pushes rows, then calls POST /get-online-features with the flag and checks the field is in the response.
• test_remote_online_store.py::test_build_online_response_feature_view_metadata: checks that RemoteOnlineStore round-trips the field into the proto.

cd sdk/python
python -m pytest tests/unit/test_feature_server_utils.py tests/unit/test_feature_server.py \
  tests/unit/infra/online_store/test_remote_online_store.py \
  -k 'FeatureViewVersionMetadata or version_metadata or feature_view_metadata'

• Before (master @ 810391f with only the tests applied): 4 failed, 2 passed.
• After: 6 passed.

All three test files: 126 passed. ruff check and ruff format --check (ruff 0.16.9) pass on the changed files. mypy adds no new errors.

Misc

The remote.py change is limited to the response-parsing helper. It should not conflict with the open PRs that touch the request and write paths of RemoteOnlineStore.

AI assistance: this change was drafted with an AI coding assistant (Claude) and verified locally with the tests above.