fix: add PHPStan baseline for nested array offset access on mixed

Webhook payloads typed as array<string, mixed> produce unavoidable
offset access errors at nested levels. Baselined for:
- ProcessWebhookEvent, GithubService, WebhookService

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tanhongit

phpstan-baseline.neon

@@ -1,5 +1,16 @@
 parameters:
     ignoreErrors:
+        - message: '#Cannot access offset .+ on mixed\.#'
+          paths:
+              - src/Jobs/ProcessWebhookEvent.php
+              - src/Services/GithubService.php
+              - src/Services/WebhookService.php
+
+        - message: '#Cannot cast mixed to (string|int)\.#'
+          paths:
+              - src/Jobs/ProcessWebhookEvent.php
+              - src/Services/GithubService.php
+
         - message: '#Parameter \#1 \$view of function view expects view-string\|null, string given\.#'
           paths:
               - src/Services/GithubService.php