fix(table): validate sort field source IDs

[fallintoplace]

Summary

• reject sort fields that omit both source-id and source-ids
• reject zero or negative sort source IDs for both JSON metadata and NewSortOrder
• add regression coverage for missing, empty, zero, negative, and mixed invalid source IDs

Why

SortField.UnmarshalJSON treated a missing source-id as the Go zero value and accepted it as source ID 0. That allowed invalid sort metadata to enter the table model and fail later during compatibility checks or other sort-order use.

Testing

• go test ./table -run 'Test(NewSortOrderRejectsInvalidSourceIDs|SortFieldMultiArgSourceIDs|UnmarshalSortOrderDefaults|UnmarshalInvalidSort)' -count=1
• go test ./table -count=1
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.4 run --timeout=10m ./table/...
• git diff --check

[zeroshade]

Thanks - validating source-id shape (presence + positivity) at unmarshal and in NewSortOrder is a solid addition, and the table tests are thorough. Requesting changes on one related gap with multi-arg source-ids - inline.

[zeroshade]

This validates that every entry in SourceIDs is present and positive - good. The remaining gap is existence in the schema: SortOrder.CheckCompatibility only looks up field.SourceID(), i.e. the first source id (sorting.go:298), so a multi-arg sort field such as SourceIDs: [1, 999] still passes when 999 isn't in the schema. Worth iterating all of field.SourceIDs there (looking each up via FindFieldByID and checking it's primitive), with a regression case pairing a valid id and a nonexistent one.