fix(util): rune-safe truncation across formatters

firstLineOf (query) and truncateString (edit) sliced strings at a raw byte
offset, splitting a multibyte UTF-8 character into invalid bytes in
LLM-facing output (ast_query compact format and edit not-found errors are
hot paths). Extract the rune-boundary backoff already used by search's
truncateLine into util.RuneBoundary and route all three sites through it.

Adds table-driven tests for the primitive and a multibyte case for firstLineOf.

internal/util/truncate.go

@@ -0,0 +1,20 @@
+package util
+
+import "unicode/utf8"
+
+// RuneBoundary returns the largest index j <= i such that s[:j] is valid UTF-8
+// (j falls on a rune start, never inside a multibyte sequence). i is clamped to
+// [0, len(s)]. Callers slicing s[:i] for truncation should slice s[:RuneBoundary(s, i)]
+// instead so a multibyte character is never split into an invalid encoding.
+func RuneBoundary(s string, i int) int {
+	if i <= 0 {
+		return 0
+	}
+	if i >= len(s) {
+		return len(s)
+	}
+	for i > 0 && !utf8.RuneStart(s[i]) {
+		i--
+	}
+	return i
+}