Improve error for nonexistent history source manifest paths

[atharva-patill]

Summary

The --history-source-manifest import path currently reports that the supplied path "did not contain" ctx-history-plugin.json even when the path does not exist.

This adds the same early path validation already used by the other explicit import paths, so nonexistent paths report the existing import path does not exist error instead.

Reproduction

./target/debug/ctx import \
  --history-source-manifest /tmp/no-such-manifest.json

Validation

• Added a regression test for nonexistent manifest paths
• Verified the new behavior manually

Before

After