schemadiff: skip keys with expressions in Online DDL analysis

[shlomi-noach]

Description

Keys may cover expressions, such as in KEY idx1 ((id + 1)). Such keys are not eligible for Online DDL (vreplication) iteration, because even if defined as UNIQUE, they cannot be trusted; the expression might un-unique the value (consider ((id * 0)) as a trivial example).

With this PR schemadiffs Online DDL analysis ignores keys that have expressions. The function IndexDefinitionEntities() likewise does not attempt to read column information from an index that has an expression.

Added various tests that would otherwise either return incorrect answer or panic with panic: runtime error: invalid memory address or nil pointer dereference.

Backport

As this can cause a crash scenario, marking for backport.

Related Issue(s)

• fixes Bug Report: schemadiff nil dereference error in Online DDL analysis #17477
• schemadiff: tracking issue #10203

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.66%. Comparing base (1ce7550) to head (498dad2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/schemadiff/onlineddl.go	25.00%	3 Missing ⚠️
go/vt/schemadiff/key.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17475   +/-   ##
=======================================
  Coverage   67.65%   67.66%           
=======================================
  Files        1584     1584           
  Lines      254409   254422   +13     
=======================================
+ Hits       172133   172167   +34     
+ Misses      82276    82255   -21     

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