fix(editor): detect SQL clause from text before the token being typed

[datlechin]

What

Fixes the macOS Tests failures on main introduced by #1653 (red since 42ecf68): "SELECT keyword returns completions" and "Replacement range does not exceed text length".

Root cause

SQLContextAnalyzer ran clause detection on the text including the token under the cursor. With SELECT| the word SELECT is both the completion prefix and, to the analyzer, a committed clause keyword. The resolved clause became .select, whose candidate set (columns, functions) contains nothing matching the prefix "select", so the engine returned zero items and getCompletions returned nil. Same failure for ... WHERE|.

Fix

Clause detection now runs on the text before the prefix token (substring(to: prefixStart)). The token being typed stays what it is: the filter prefix. Typing SELE or a full SELECT keeps suggesting the SELECT keyword; positions with a trailing space (JOIN |, FROM users |) are unchanged, so the table suggestions #1653 added still work.

One test from #1653 placed the cursor touching the end of FROM| and asserted .from. Its intent (block comments do not break clause detection) is preserved by moving the cursor after the trailing space, where FROM is committed.

Testing

All five autocomplete suites pass locally (698 cases): CompletionEngineTests, CompletionEngineFilterTests, SQLClauseDetectionTests, SQLContextAnalyzerTests, SQLCompletionProviderTests. The three remaining local failures (testMySQLProviderTypes, testProviderAcceptsDatabaseType, testCommaFromScopesColumnsToAllTables) fail identically on clean main without this change; they need driver plugins present and pass on CI.

No CHANGELOG entry: this fixes behavior that is itself still unreleased (#1653).