Address review: use typedLeader for insertText/label, extract utils

Use typedLeader instead of hardcoded chatVariableLeader for insertText
and label in selection, file/folder, symbol, and tool completions so
that @ is fully equivalent to # as a context reference leader.

Extract escapeForCharClass, computeCompletionRanges, and
isEmptyUpToCompletionWord into chatInputCompletionUtils.ts to allow
lightweight unit testing without pulling in the full completions module.

Co-authored-by: Copilot <[email protected]>