fix(frontend): explain should return distributed query plan

[liurenjie1024]

What's changed and what's your intention?

RT

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests

Refer to a related PR or issue link (optional)

Closes #2037

[codecov]

Codecov Report

Merging #2103 (aa19345) into main (400dca9) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #2103   +/-   ##
=======================================
  Coverage   70.66%   70.66%           
=======================================
  Files         636      636           
  Lines       80718    80718           
=======================================
  Hits        57037    57037           
  Misses      23681    23681           

Flag	Coverage Δ
rust	70.66% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/frontend/src/handler/explain.rs	0.00% <0.00%> (ø)
src/meta/src/stream/stream_manager.rs	63.32% <0.00%> (-0.32%)	⬇️
.../src/executor/managed_state/aggregation/extreme.rs	90.29% <0.00%> (+0.26%)	⬆️

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more