Fix dryrun namespace derivation to match push behavior

[shangyian]

Summary

dj push --dryrun without --namespace flag used the top-level namespace from dj.yaml instead of deriving the branch-qualified namespace (e.g., xyz.main). This caused the server to process the wrong namespace, leading to hangs or incorrect impact analysis.

The fix applies the same namespace derivation logic that push already uses.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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