[ES|QL] improve support for "invoke" autocomplete triggers

[drewdaemon]

Describe the feature: In some cases, our autocomplete refuses to produce suggestions when a user starts or continues typing a word. It can also appear not to take into account the characters that have already been typed in the current word. A user must often delete one or more characters and press the space bar to get the suggestions menu open again.

https://github.com/user-attachments/assets/f12ff0ae-6b2f-47f5-9fc1-37555308ab95

This is confusing behavior and often leaves the user without good suggestions.

Why this happens

Monaco produces suggestions based on various events. Two kinds of events are "Invoke" and "Trigger character" (ref). "Trigger character" means the user enters one of a set of special characters we have configured to pop the suggestions menu open (e.g. ' '). "Invoke" means the user simply starts typing a word.

The inconsistencies in our autocomplete behavior are largely symptomatic of spotty support for "Invoke" style events.

	Invoke (0)	Trigger character (1)
Source command	✅	✅
Pipe command	❌	✅
Function argument	✅	✅
FROM source	❌	✅
FROM source METADATA	✅	✅
FROM source METADATA field	❌ (for _)	✅
EVAL argument	✅	✅
DISSECT field	✅	✅
DISSECT field pattern	❌	✅
DROP (first field)	✅	✅
DROP (subsequent fields)	❌	✅
ENRICH policy	✅	✅
ENRICH policy ON	✅	✅
ENRICH policy ON field	✅	✅
ENRICH policy WITH	✅	✅
ENRICH policy WITH field	✅	✅
GROK field	✅	✅
GROK field pattern	❌	✅
KEEP field	✅	✅
KEEP field1, field2	❌	✅
LIMIT number	❌	✅
MV_EXPAND field	✅	✅
RENAME field	✅	✅
RENAME field AS	❌	✅
RENAME field AS var0	✅	✅
SORT field	✅	✅
SORT field order	✅	✅
SORT field order nulls-order	✅	✅
STATS argument	✅	✅
STATS argument BY	✅	✅
STATS argument BY expression	✅	✅
WHERE argument	✅	✅
WHERE argument comparison	✅ (partial, not with special characters)	✅

[elasticmachine]

Pinging @elastic/kibana-esql (Team:ESQL)