fix: guard Tia argv scanning against non-string args (#1736)

The Tia plugin scans the raw argv with str_starts_with() in
argumentPresent() and hasExplicitPathArgument(). In the parallel worker
path (bin/worker.php) the unserialized argv can contain an integer
--random-order-seed value as a separate element, which made
str_starts_with() throw:

    TypeError: str_starts_with(): Argument #1 ($haystack) must be of
    type string, int given

This is the same class of bug as #1206, which was only fixed in
HandleArguments; it resurfaced in the newer Tia plugin. Cast each argv
element to string before scanning, mirroring the #1206 fix.

Adds a regression test, and updates the parallel count assertion and the
success snapshot to account for it.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/Plugins/Tia.php

@@ -251,7 +251,7 @@ final class Tia implements AddsOutput, HandlesArguments, Terminable
                 return true;
             }
 
-            if (str_starts_with($arg, "$argument=")) {
+            if (str_starts_with((string) $arg, "$argument=")) { // @phpstan-ignore-line
                 return true;
             }
         }
@@ -1489,6 +1489,8 @@ final class Tia implements AddsOutput, HandlesArguments, Terminable
         }
 
         foreach ($arguments as $index => $arg) {
+            $arg = (string) $arg; // @phpstan-ignore-line
+
             if ($arg === '') {
                 continue;
             }