Fail assignment expressions with the correct message

[GuptaManan100]

Description

As pointed out in #17751, we shoud be failing assignment expressions with the proper error message.

Related Issue(s)

• Fixes Bug Report: Assignment Expressions fail with a parser error #17751

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[dbussink]

@GuptaManan100 Hmm, this now fails in the parser tests:

--- FAIL: TestNormalizeValidSQL (0.05s)
    --- FAIL: TestNormalizeValidSQL/update_a.b_set_d_=_@v_:=_d_+_7_where_u_=_42 (0.00s)
        normalizer_test.go:506: 
            	Error Trace:	/home/runner/work/vitess/vitess/go/vt/sqlparser/normalizer_test.go:506
            	Error:      	Received unexpected error:
            	            	VT12001: unsupported: Assignment expression
            	Test:       	TestNormalizeValidSQL/update_a.b_set_d_=_@v_:=_d_+_7_where_u_=_42

I wonder if that indicates we should do this check only later on? When we try to plan it? Or is this the right place to handle it?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.96%. Comparing base (c47f1bd) to head (1da6722).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17752   +/-   ##
=======================================
  Coverage   67.95%   67.96%           
=======================================
  Files        1586     1585    -1     
  Lines      255227   255165   -62     
=======================================
- Hits       173445   173413   -32     
+ Misses      81782    81752   -30     

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