test(metadata): cover metadata exclusion rules in split_metadata()

[GITHUBTANMAY2003]

Description

Adds regression coverage for metadata exclusion behavior in split_metadata(). These tests ensure that non-filterable metadata is excluded from S3 Vectors while remaining available in DynamoDB, and that required namespace metadata is always preserved.

This is a test-only change; no production code was modified.

Related issue

Fixes #245

Changes

• Added coverage verifying that non_filterable_keys are excluded from S3 metadata but retained in DynamoDB.
• Added coverage verifying denylist precedence and mandatory namespace preservation.
• Added coverage verifying that split_metadata() does not mutate caller-provided metadata.

Testing

• Tests pass locally
• Ruff checks pass
• Documentation updated, if applicable

Verification performed:

• Four focused regression tests: 4 passed
• Complete metadata test module: 29 passed
• Full offline test suite: passed
• make run-ci: passed
• Tests verified without live AWS credentials
• Two live-AWS tests skipped as expected

Checklist

• My changes are focused and relevant to this pull request.
• I have added or updated tests where appropriate.
• I have reviewed my changes for unrelated modifications.
• I have updated documentation where necessary.

[Kaap10]

LGTM! Excellent work @GITHUBTANMAY2003. All 4 metadata exclusion test scenarios are cleanly covered and tested offline.