test(tools,utils): add ToolRegistry unit tests and fix Truncate panic on negative maxLen (#517)

Add comprehensive unit tests for the ToolRegistry covering registration,
lookup, execution, context injection, async callbacks, schema generation,
provider definition conversion, and concurrent access.

Fix a defensive edge case in Truncate where a negative maxLen would cause
a slice bounds panic, and add table-driven tests covering boundary
conditions, zero/negative lengths, and Unicode handling.

Co-authored-by: Cursor <cursoragent@cursor.com>

pkg/utils/string.go

@@ -4,6 +4,9 @@ package utils
 // Handles multi-byte Unicode characters properly.
 // If the string is truncated, "..." is appended to indicate truncation.
 func Truncate(s string, maxLen int) string {
+	if maxLen <= 0 {
+		return ""
+	}
 	runes := []rune(s)
 	if len(runes) <= maxLen {
 		return s