fix(requirements): validate missing last partition spec id

[fallintoplace]

Summary

Fix a panic in requirement validation when metadata lacks last partition spec ID (for example v1 or malformed metadata).

The previous implementation directly dereferenced meta.LastPartitionSpecID() and could panic. This change now checks for nil and returns a clean requirement failure message:

• requirement failed: last assigned partition id is missing

When the field is present, behavior is unchanged and the previous comparison path still validates for mismatches.

Testing

• go test ./table -run TestAssertLastAssignedPartitionIDValidate -count=1

[zeroshade]

LGTM. The nil LastPartitionSpecID case now returns a requirement failure instead of panicking, consistent with the other requirement validators, and the tests cover missing/match/mismatch.