#334: remove JSON as recommended codec for new v2 workflows

BackendCapabilities: diagnostic messages no longer suggest json as
an option for new runs — only avro.

CodecRegistry::engineSpecific(): json excluded from php-specific list
(it is language-neutral, kept for decode only).

Tests updated to reflect json is no longer universal in v2.

Author: durable-workflow-ops

src/Serializers/CodecRegistry.php

@@ -139,7 +139,10 @@ public static function engineSpecific(): array
     {
         $universal = self::universal();
 
-        $phpOnly = array_values(array_diff(array_keys(self::CODECS), $universal));
+        // Exclude both universal codecs and json (language-neutral, kept for
+        // decode of existing data but not advertised as engine-specific).
+        $excluded = array_merge($universal, ['json']);
+        $phpOnly = array_values(array_diff(array_keys(self::CODECS), $excluded));
 
         if ($phpOnly === []) {
             return [];

src/V2/Support/BackendCapabilities.php

@@ -279,7 +279,7 @@ private static function codec(): array
                         .'("%s") and the legacy PHP-only codecs ("workflow-serializer-y", "workflow-serializer-base64") '
                         .'are resolvable. '
                         .'To migrate a v1 deployment that used a custom serializer, choose one of: '
-                        .'(a) set workflows.serializer to "avro" or "json" for new runs and accept that '
+                        .'(a) set workflows.serializer to "avro" for new runs and accept that '
                         .'old history written under the custom codec becomes unreadable, '
                         .'(b) keep the custom serializer classes loaded and re-encode old history to a '
                         .'supported codec before upgrading, '
@@ -304,7 +304,7 @@ private static function codec(): array
                 'warning',
                 'codec_legacy_php_only',
                 sprintf(
-                    'workflows.serializer is set to [%s], a PHP-only legacy codec. New v2 workflows written with this codec cannot be decoded by Python, Go, or TypeScript workers. Set workflows.serializer to "json" for polyglot compatibility; keep the legacy codec only if you still need to read v1 history with it.',
+                    'workflows.serializer is set to [%s], a PHP-only legacy codec. New v2 workflows written with this codec cannot be decoded by Python, Go, or TypeScript workers. Set workflows.serializer to "avro" for polyglot compatibility; keep the legacy codec only if you still need to read v1 history with it.',
                     $canonical,
                 ),
             );

tests/Unit/V2/BackendCapabilitiesTest.php

@@ -208,19 +208,15 @@ public function testQueueModeStillRejectsSyncQueueDriver(): void
         $this->assertSame('error', $queueIssue['severity']);
     }
 
-    public function testDefaultJsonCodecIsUniversalAndHasNoIssues(): void
+    public function testJsonCodecIsNotUniversalInV2(): void
     {
         config()->set('workflows.serializer', 'json');
 
         $snapshot = BackendCapabilities::snapshot();
 
         $this->assertSame('json', $snapshot['codec']['canonical']);
-        $this->assertTrue($snapshot['codec']['universal']);
+        $this->assertFalse($snapshot['codec']['universal']);
         $this->assertTrue($snapshot['codec']['supported']);
-        $this->assertEmpty($snapshot['codec']['issues']);
-        $this->assertNull(
-            collect($snapshot['issues'])->firstWhere('component', 'codec'),
-        );
     }
 
     public function testLegacyPhpCodecEmitsPolyglotCompatibilityWarning(): void
@@ -281,7 +277,7 @@ public function testUnknownCodecDiagnosticIncludesMigrationGuidance(): void
 
         // It must name the universal codec options an operator can migrate to.
         $this->assertStringContainsString('avro', $codecIssue['message']);
-        $this->assertStringContainsString('json', $codecIssue['message']);
+        // json is no longer recommended for new workflows (Avro-only per #334)
 
         // It must mention that default-codec resolution silently falls back
         // to avro so operators understand why new runs still work — and that