ESQL: Re-enable dependency checks

[costin]

Description

Re-enable the optimizers dependency checkers - see:
#105371 (comment)

[elasticsearchmachine]

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

[alex-spies]

When re-enabling, we should make sure that a failed dependency check results in a 500 response code sent back to the user; therefore, we can't throw a VerificationException or any other client exception - we could throw an IllegalStateException instead. See discussion here.