refactor: improve error handling code quality

Address code review feedback from Gemini Code Assist:

1. Reduce duplication in ClaudeProvider catch block
   - Consolidate error creation logic into single path
   - Use conditional message building instead of duplicate blocks
   - Improves maintainability and follows DRY principle

2. Better separation of concerns in error utilities
   - Move default retry-after (60s) logic from extractRetryAfter to classifyError
   - extractRetryAfter now only extracts explicit values
   - classifyError provides default using nullish coalescing (?? 60)
   - Clearer single responsibility for each function

3. Update test to match new behavior
   - extractRetryAfter now returns undefined for rate limits without explicit value
   - Default value is tested in classifyError tests instead

All 162 tests still passing ✅
Builds successfully with no TypeScript errors ✅

libs/utils/tests/error-handler.test.ts

@@ -145,9 +145,9 @@ describe('error-handler.ts', () => {
       expect(extractRetryAfter(error)).toBe(30);
     });
 
-    it('should return default 60 for rate limit errors without explicit retry-after', () => {
+    it('should return undefined for rate limit errors without explicit retry-after', () => {
       const error = new Error('429 rate_limit_error');
-      expect(extractRetryAfter(error)).toBe(60);
+      expect(extractRetryAfter(error)).toBeUndefined();
     });
 
     it('should return undefined for non-rate-limit errors', () => {