fix: handle nil jsonpointer sources

[kv1sidisi]

Summary

• return a not found error when jsonpointer navigation receives an untyped nil source
• add a regression test for nil source navigation

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Summary by cubic

Return a "not found" error when jsonpointer navigation receives an untyped nil source, with a clear path message. Adds a regression test to cover nil source traversal.

^{Written for commit 5b7d9d3. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}