fix(planner): mocksource emit name for unnest

[Yisaer]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.81%. Comparing base (5dee73c) to head (8daf716).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4008      +/-   ##
==========================================
- Coverage   71.87%   71.81%   -0.07%     
==========================================
  Files         460      460              
  Lines       53183    53190       +7     
==========================================
- Hits        38224    38194      -30     
- Misses      12024    12054      +30     
- Partials     2935     2942       +7     

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[ngjaying]

LGTM. This perfectly aligns with the finding that while the $ prefix is necessary to prevent global schema store corruption and topology ID collisions during rule tests, it shouldn't leak as the emitter name into the downstream data flow. If it does, operators like unnest or field projections that expect the original stream name will fail to find their fields. Explicitly adding an EmitterOp to restore the emitter name while keeping the internal node name mocked is the correct fix.