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>
King Tai