What this PR does / why we need it:

S3RegistryStore already reads FEAST_S3_ENDPOINT_URL and hands it to boto3, so the registry can live on an S3-compatible object store, but the variable is not mentioned anywhere in the registry reference docs. The repo already depends on it in practice (examples/python-helm-demo/minio.env, the Helm example's Kubernetes env patch, and the MinIO container in test_universal_registry.py), so this adds a short section to docs/reference/registries/s3.md covering the variable and the credentials boto3 resolves alongside it.

It also sets the Feast user agent on the S3 registry's boto3 client. Feast already tags its BigQuery, Datastore, DynamoDB, Redshift and Athena clients with get_user_agent(), and the S3 registry client was the remaining gap. This passes it as user_agent_extra rather than user_agent so the Feast tag is appended to botocore's default string instead of replacing it, which keeps the botocore, OS and Python metadata intact:

Boto3/1.43.22 md/Botocore#1.43.22 ua/2.1 os/macos#25.6.0 md/arch#arm64 lang/python#3.11.15 md/pyimpl#CPython m/N,g Botocore/1.43.22 feast-dev/feast/0.65.0

The tag reuses the existing feast.utils.get_user_agent() helper, whose version already resolves from package metadata with an "unknown" fallback. No new config keys, no new dependencies, and no behavior change for existing users.

Which issue(s) this PR fixes:

None.

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

Misc

ruff check and ruff format --check are clean on sdk/python/feast/infra/registry/s3.py, and mypy feast/infra/registry/s3.py reports nothing in that file.

I ran tests/unit/test_registry_string_config.py, the only unit file that references S3RegistryStore, and test_s3_scheme_selects_s3_registry_store passes. The other failures in that file come from a missing optional redis extra in my environment and reproduce identically on a clean master, so they are unrelated to this change.

I also confirmed by hand that S3RegistryStore still honors FEAST_S3_ENDPOINT_URL and that the resolved user agent keeps botocore's default prefix with feast-dev/feast/<version> appended (the string quoted above). Happy to split the docs and client changes into separate PRs, or to switch to user_agent= for consistency with the other clients, if you'd prefer either.