[kvexec] fix more lookup bugs related to schema/projection inconsistencies

[max-hoffman]

We were using the full table schema to index a covering index KV pair. The fix has to be sensitive to covering, non-covering, and keyless tables, which required extra tests on the GMS side to check all cases.

fixes: #8308
re: dolthub/go-mysql-server#2646

[zachmu]

Code looks fine, but where are the tests?

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
0f40db9	ok	5937457

version	total_tests
0f40db9	5937457

correctness_percentage
100.0

[max-hoffman]

Code looks fine, but where are the tests?

GMS side, linked in description dolthub/go-mysql-server#2646

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
485313c	ok	5937457

version	total_tests
485313c	5937457

correctness_percentage
100.0

[coffeegoddd]

@max-hoffman DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
5481ace	ok	5937457

version	total_tests
5481ace	5937457

correctness_percentage
100.0