fix(plan): support modern multi-target updates

[ck89119]

What type of PR is this?

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

Which issue(s) this PR fixes:

fixes #26340

What this PR does / why we need it:

Multi-target UPDATE statements were still routed to the legacy planner because
the modern path could not independently deduplicate and filter rows for each
updated table. A shared source row may represent a valid target row for one
table and a duplicate or outer-join NULL row for another.

This change:

• routes multi-target UPDATE statements through the modern MULTI_UPDATE
  path;
• adds an independent row_number() selector for each physical target Rowid;
• shares each target's filtered batch with its base, unique-index, and
  secondary-index write contexts in both table and S3 writers;
• guards primary-key and unique-index DEDUP keys so unselected duplicate source
  rows do not trigger false duplicate-key errors;
• preserves valid rows from other targets when a multi-target outer join has no
  old secondary-index row for one target;
• carries the selector ownership through protobuf, remote execution, deep-copy,
  and partition-clone paths; and
• adds planner, executor, serialization, partition, and distributed regression
  coverage.

Validation:

• mo-cgo-test ./pkg/sql/plan -count=1
• mo-cgo-test ./pkg/sql/colexec/multi_update -count=1
• mo-cgo-test ./pkg/sql/compile -count=1
• mo-cgo-test ./pkg/sql/plan -race -count=1
• isolated mo-tester BVT: 17/17 statements passed
• make build
• make static-check
• git diff --check