fix: OpenAI StructuredModeResolver should match versioned model names

StructuredModeResolver::supportsStructuredMode() uses in_array with
exact string matching. Versioned model names like gpt-5-mini-2025-08-07
don't match gpt-5-mini, causing them to silently fall into JSON mode
(json_object) instead of Structured mode (json_schema + strict).

In JSON mode, the schema is embedded as plain text in a system message
with no constrained decoding, resulting in significantly more verbose
model output.

Use a hybrid approach:
- Keep exact matching for gpt-4o and o3-mini families where only
  specific versions support structured output
- Use prefix matching for gpt-4.1+, gpt-4.5+, and gpt-5+ families
  where structured output is a baseline feature across all versions

Author: ipalaus

src/Providers/OpenAI/Support/StructuredModeResolver.php

@@ -24,26 +24,34 @@ public static function forModel(string $model): StructuredMode
 
     protected static function supportsStructuredMode(string $model): bool
     {
-        return in_array($model, [
+        // Exact matches for models where only specific versions support structured output.
+        if (in_array($model, [
             'gpt-4o-mini',
             'gpt-4o-mini-2024-07-18',
             'gpt-4o-2024-08-06',
             'gpt-4o',
             'chatgpt-4o-latest',
             'o3-mini',
             'o3-mini-2025-01-31',
+        ])) {
+            return true;
+        }
+
+        // Prefix matching for model families where structured output is a baseline feature.
+        // All versions of these families support structured mode.
+        $prefixes = [
             'gpt-4.1',
-            'gpt-4.1-nano',
-            'gpt-4.1-mini',
-            'gpt-4.5-preview',
-            'gpt-4.5-preview-2025-02-27',
+            'gpt-4.5',
             'gpt-5',
-            'gpt-5-mini',
-            'gpt-5-nano',
-            'gpt-5.1',
-            'gpt-5.2',
-            'gpt-5.4',
-        ]);
+        ];
+
+        foreach ($prefixes as $prefix) {
+            if (str_starts_with($model, $prefix)) {
+                return true;
+            }
+        }
+
+        return false;
     }
 
     protected static function supportsJsonMode(string $model): bool

tests/Providers/OpenAI/Support/StructuredModeResolverTest.php

@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Providers\OpenAI\Support;
+
+use Prism\Prism\Enums\StructuredMode;
+use Prism\Prism\Exceptions\PrismException;
+use Prism\Prism\Providers\OpenAI\Support\StructuredModeResolver;
+
+it('resolves structured mode for exact-match models', function (string $model): void {
+    expect(StructuredModeResolver::forModel($model))->toBe(StructuredMode::Structured);
+})->with([
+    'gpt-4o',
+    'gpt-4o-mini',
+    'gpt-4o-mini-2024-07-18',
+    'gpt-4o-2024-08-06',
+    'chatgpt-4o-latest',
+    'o3-mini',
+    'o3-mini-2025-01-31',
+]);
+
+it('resolves structured mode for gpt-4.1+ family models', function (string $model): void {
+    expect(StructuredModeResolver::forModel($model))->toBe(StructuredMode::Structured);
+})->with([
+    'gpt-4.1',
+    'gpt-4.1-nano',
+    'gpt-4.1-mini',
+    'gpt-4.1-nano-2025-04-14',
+    'gpt-4.1-mini-2025-04-14',
+    'gpt-4.5-preview',
+    'gpt-4.5-preview-2025-02-27',
+]);
+
+it('resolves structured mode for gpt-5 family models', function (string $model): void {
+    expect(StructuredModeResolver::forModel($model))->toBe(StructuredMode::Structured);
+})->with([
+    'gpt-5',
+    'gpt-5-mini',
+    'gpt-5-nano',
+    'gpt-5-mini-2025-08-07',
+    'gpt-5-nano-2025-08-07',
+    'gpt-5.1',
+    'gpt-5.1-2025-10-01',
+    'gpt-5.2',
+    'gpt-5.2-2025-12-11',
+    'gpt-5.4',
+]);
+
+it('resolves json mode for models without structured support', function (string $model): void {
+    expect(StructuredModeResolver::forModel($model))->toBe(StructuredMode::Json);
+})->with([
+    'gpt-4-turbo',
+    'gpt-4-0125-preview',
+    'gpt-3.5-turbo',
+    'some-custom-model',
+]);
+
+it('throws for unsupported models', function (string $model): void {
+    StructuredModeResolver::forModel($model);
+})->with([
+    'o1-mini',
+    'o1-mini-2024-09-12',
+    'o1-preview',
+    'o1-preview-2024-09-12',
+])->throws(PrismException::class);