ES|QL: Resolve Keep plan added to FORK branches

[ioanatia]

fixes #129715

In order to align the output of FORK branches, we always add an explicit Keep plan at end of each branch.
However, we never actually resolve the KEEP plans we are adding.
KEEP can be unresolved in case it contains columns with unsupported or conflicting types.

Then we end up skipping validations in the verifier because the KEEP is unresolved:

elasticsearch/x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/analysis/Verifier.java

Lines 109 to 113 in b2acda9

	plan.forEachDown(p -> {
	// if the children are unresolved, so will this node; counting it will only add noise
	if (p.childrenResolved() == false) {
	return;
	}

[ChrisHegarty]

LGTM

[tteofili]

good catch, LGTM

[luigidellaquila]

LGTM, thanks @ioanatia !

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)