fix(completion): use InsertAsSnippet for query language commands

[Blankll]

Summary

• Fix snippet placeholder ($0) leaking into query language command completions
• Change insertTextRules from None to InsertAsSnippet for command keywords

Problem

When accepting completions for query language commands (ESQL, SQL, EQL, PPL) inside the query body field of _query, _sql, _eql/search, and _plugins/_ppl endpoints, the literal text $0 was inserted into the editor.

For example, completing WHERE produced WHERE $0 instead of WHERE with cursor positioned after it.

Root Cause

The command definitions use Monaco snippet placeholders in their insertText:

{ label: 'WHERE', insertText: 'WHERE $0', ... }

But the completion provider used InsertTextRule.None, which inserts text literally without interpreting snippet syntax.

Fix

Changed insertTextRules at completionProvider.ts:764:

- insertTextRules: monaco.languages.CompletionItemInsertTextRule.None,
+ insertTextRules: monaco.languages.CompletionItemInsertTextRule.InsertAsSnippet,

This affects all four query languages: ESQL, SQL, EQL, and PPL.

Verification

• ✅ ESLint clean
• ✅ TypeScript type check clean
• ✅ No other instances of this bug in the codebase (audited all insertTextRules usages)

Refers: #412

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.68%. Comparing base (01307b8) to head (d8a6967).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #416   +/-   ##
=======================================
  Coverage   64.68%   64.68%           
=======================================
  Files          47       47           
  Lines        2308     2308           
  Branches      597      597           
=======================================
  Hits         1493     1493           
  Misses        698      698           
  Partials      117      117           

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