What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #26548

What this PR does / why we need it:

The MySQL 1093 compatibility check treated every direct read of an UPDATE target table inside a subquery as an uncorrelated self-read. This regressed the correlated UPDATE behavior previously verified in #24824.

This change makes the check scope-aware:

• correlated target-table subqueries are allowed when they reference the visible outer UPDATE target name or alias;
• update-target qualifiers are tied to the table occurrence actually modified by SET, so read-only self-join aliases cannot grant a correlation exemption;
• target context is preserved through nested JOIN ON expressions and SELECT wrapper clauses such as ORDER BY, LIMIT, and TimeWindow;
• JOIN ON qualifier visibility follows the current join's left and right inputs, so later FROM aliases cannot shadow outer target references prematurely;
• CROSS APPLY table-function arguments participate in outer-target correlation analysis while left-input aliases retain lateral shadowing semantics;
• local aliases correctly shadow outer target qualifiers across nested SELECT scopes;
• UNION branches retain independent correlation decisions while wrapper-level correlation applies to the complete query block;
• uncorrelated target-table reads continue to return error 1093;
• target identity remains isolated for same-named tables from different schemas.

The check remains planner-only and adds no execution-path overhead.

Validation:

• GOWORK=off go list -mod=readonly ./pkg/sql/plan
• GOWORK=off go build -mod=readonly ./pkg/sql/plan
• GOWORK=off go vet -mod=readonly ./pkg/sql/plan
• golangci-lint run -c .golangci.yml ./pkg/sql/plan/...
• .agents/skills/mo-dev/scripts/mo-cgo-test -count=1 -timeout=120s ./pkg/sql/plan
• changed production statement coverage: 176/218 = 80.73%