fix(expr): fix casting rules for struct

[kwannoel]

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What's changed and what's your intention?

Fix row type alignment. As mentioned by @xiangjinwu here:

It is due to the fact that we do not allow casting between the same type, so the 2nd field fails to match interval to interval 😂

Thanks to @xiangjinwu for pointing out the quick fix.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Documentation

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Types of user-facing changes

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• Installation and deployment
• Connector (sources & sinks)
• SQL commands, functions, and operators
• RisingWave cluster configuration changes
• Other (please specify in the release note below)

Release note

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Refer to a related PR or issue link (optional)

Closes #7189

[codecov]

Codecov Report

Merging #7202 (4afbe2d) into main (80abae3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #7202      +/-   ##
==========================================
+ Coverage   73.13%   73.14%   +0.01%     
==========================================
  Files        1055     1055              
  Lines      168781   168781              
==========================================
+ Hits       123430   123454      +24     
+ Misses      45351    45327      -24     

Flag	Coverage Δ
rust	73.14% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/frontend/src/expr/type_inference/cast.rs	94.97% <100.00%> (+4.56%)	⬆️
src/storage/src/hummock/compactor/sstable_store.rs	74.54% <0.00%> (-0.91%)	⬇️
src/meta/src/hummock/mock_hummock_meta_client.rs	65.26% <0.00%> (-0.53%)	⬇️
src/object_store/src/object/mem.rs	87.12% <0.00%> (-0.38%)	⬇️
src/storage/src/hummock/compactor/iterator.rs	98.02% <0.00%> (-0.29%)	⬇️
src/meta/src/hummock/manager/mod.rs	79.39% <0.00%> (-0.06%)	⬇️
src/frontend/src/binder/select.rs	88.92% <0.00%> (+0.32%)	⬆️
src/frontend/src/expr/function_call.rs	87.82% <0.00%> (+7.82%)	⬆️

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[xiangjinwu]

Maybe also add to planner test, where we can see the computed common type and inserted cast.