refactor: improve type safety and code quality for 9.0 score

Type Safety Improvements:
- Add isErrorResult() type guard for API responses (domain-tool.ts)
- Replace `any` with `unknown` in executeTool args (tools/index.ts)
- Add JSON.parse error handling in function calling (openai-service.ts)
- Fix nullable price handling with nullish coalescing
- Add array type guard for nameservers validation

Code Quality Improvements:
- Extract convertNamecheapDate() to eliminate duplicate functions
- Move hardcoded bank account info to environment variables
- Add JSDoc documentation to executeDepositFunction
- Fix unused variables in optimistic-lock.ts
- Handle Error.captureStackTrace for Workers environment

All TypeScript strict mode checks now pass.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/openai-service.ts

@@ -130,7 +130,16 @@ export async function generateOpenAIResponse(
         // 도구 호출 결과 수집
         const toolResults: OpenAIMessage[] = [];
         for (const toolCall of assistantMessage.tool_calls) {
-          const args = JSON.parse(toolCall.function.arguments);
+          let args: Record<string, unknown>;
+          try {
+            args = JSON.parse(toolCall.function.arguments);
+          } catch (parseError) {
+            logger.error('Failed to parse tool arguments', parseError as Error, {
+              toolName: toolCall.function.name,
+              raw: toolCall.function.arguments.slice(0, 200) // 일부만 로깅
+            });
+            continue; // 다음 tool call로 진행
+          }
           const result = await executeTool(toolCall.function.name, args, env, telegramUserId, db);
 
           // __KEYBOARD__ 마커가 있으면 AI 재해석 없이 바로 반환 (버튼 보존)