fix(lsp): a panic caused by completion with decl_id out of range (#15576)

Fixes a bug caused by #15536 
Sorry about that, @fdncred 

# Description

I've made the panic reproducible in the test case.

TLDR: completer will sometimes return new decl_ids outside of the range
of the engine_state passed in.

# User-Facing Changes

bug fix

# Tests + Formatting

+1

# After Submitting

tests/fixtures/lsp/completion/command.nu

@@ -10,3 +10,5 @@ def "config n foo bar" [
   echo "🤔🐘"
   | str substring (str substring -)
 }
+
+config n # don't panic!