feat: integrate rate limit store, upgrade WebhookTester, API key creation flow, and settings security validation

[OlolaJaco]

Description

This PR resolves several key issues across the Developer tools and Merchant Settings sections:                                                                                       
                                                                                                                                                                                     
1. **Rate Limit Store Integration & Display (Closes #323)**:                                                                                                                         
   - Updated `useRateLimitStore` (`lib/store/rateLimitStore.ts`) to track the rate-limited endpoint URL and rate limit policy (max requests allowed per window).                     
   - Updated the `axios` response interceptor (`lib/api/axios.ts`) to capture 429 rate limit details (`retry-after` header, endpoint URL, `X-RateLimit-Limit`) and update            

useRateLimitStore.
- Connected RateLimitDisplay (components/developers/RateLimitDisplay.tsx) to subscribe to useRateLimitStore.
- Displays an active HTTP 429 banner featuring:
- Countdown timer with remaining seconds (formatted as HH:MM:SS and Xs remaining), driven by the store's tick() function.
- Rate-limited endpoint URL.
- Rate limit policy (max requests per window).
- Added unit tests in RateLimitDisplay.test.tsx validating 429 state display and timer countdown logic.

2. **Authenticated Webhook Tester (Closes #322)**:                                                                                                                                   
   - Upgraded `WebhookTester` (`components/developers/WebhookTester.tsx`) to send real HTTP requests via `apiClient`.                                                                
   - Added options to configure custom webhook target endpoints, custom headers, and custom JSON payload structures.                                                                 
   - Integrated webhook HMAC signature headers (`X-Webhook-Signature`) and payload delivery status reporting.                                                                        
   - Added unit tests in `WebhookTester.test.tsx`.                                                                                                                                   
                                                                                                                                                                                     
3. **Secure API Key Generation Flow (Closes #321)**:                                                                                                                                 
   - Implemented real API key creation in `app/(merchant)/developers/page.tsx` posting to `/api/keys`.                                                                               
   - Added a "Save Your API Key" modal displaying the full secret API key once upon creation with explicit copy functionality and security warnings.                                 
   - Added unit tests in `page.test.tsx` verifying key creation and success dialog interactions.                                                                                     
                                                                                                                                                                                     
4. **Settings Page Security & Password Validation(Closes #320 )**:                                                                                                                                 
   - Added client-side password validation logic (current password, new password complexity criteria, and password confirmation matching) in `app/(merchant)/settings/page.tsx`.     
   - Added comprehensive unit tests in `securityValidation.test.tsx`.                                                                                                                
                                                                                                                                                                                     
---                                                                                                                                                                                  
                                                                                                                                                                                     
## Issues Closed                                                                                                                                                                     

- Closes #323
- Closes #322
- Closes #321
- Closes #320 

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