ESQL: prevent usage of BUCKET in agg filters

[bpintea]

Using BUCKET in agg filters should not be allowed (as it's a grouping function). See example here.

Related #113735

[elasticsearchmachine]

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

[bpintea]

This has been already working as intended: reuse of expression from grouping allowed, but other not. Adding testing in #117184.