fix(editor): replace the whole typed word when accepting a completion

[datlechin]

Problem

User report: with a column named message, typing mess and pressing Tab inserts memessage instead of message. The completion left the first typed characters in place and replaced only a tail subrange.

Root cause

SQLCompletionAdapter.completionWindowApplyCompletion built the replace range from a stored token start paired with the live cursor:

• the start was computed asynchronously when the suggestion window opened (50ms debounce, actor hops, on-demand schema fetches, 5,000-char window offset, statement-offset arithmetic)
• completionOnCursorMove kept the window open across keystrokes but never updated the stored range
• the fuzzy filter let candidates survive a corrupted effective prefix (ss fuzzy-matches message), so nothing self-corrected before Tab

Any drift in that pipeline became a wrong location, and Tab replaced the wrong subrange. The happy path was correct, which is why the bug appeared only intermittently (fast typing during a schema fetch, multi-statement or multibyte documents).

Fix

Compute the replacement range from the live text at accept time, the way AppKit's own completion (rangeForUserCompletion) behaves:

• new SQLTokenBoundary holds the single identifier-boundary rule (UTF-16 scanning, stops at . so qualified-name segment completion keeps its semantics) and replacementRange(in:cursor:fallback:)
• completionWindowApplyCompletion and completionOnCursorMove both use it, so the visible filtering and the applied range always agree; the stored range remains only as a fallback when no cursor is supplied
• SQLContextAnalyzer delegates its boundary checks to the shared rule so analyzer and adapter cannot diverge again

Tests

11 unit tests in SQLTokenBoundaryTests: plain word, dot segments, quoted identifiers, multibyte text before the token, cursor clamping, empty prefix, and the regression case (stale stored range + live cursor must produce SELECT message, never SELECT memessage). All pass locally; swiftlint lint --strict clean on changed files.