fix(ffe): Correct error code mapping and enforce ERROR reason for non-zero error codes

- Fix errorCodeToTag() to match actual Rust constants:
  1=ERROR_TYPE_MISMATCH→type_mismatch, 2=ERROR_CONFIG_PARSE→parse_error,
  3=ERROR_FLAG_UNRECOGNIZED→flag_not_found (was transposed)
- Per OpenFeature spec, force reason=ERROR for any non-zero error_code.
  FlagUnrecognizedOrDisabled returns REASON_DEFAULT from the Rust layer
  but must be reported as ERROR to callers.

Author: leoromanovsky

src/DDTrace/FeatureFlags/FlagEvalMetrics.php

@@ -103,11 +103,11 @@ private static function errorCodeToTag($code)
     {
         switch ((int)$code) {
             case 1:
-                return 'flag_not_found';
+                return 'type_mismatch';    // ERROR_TYPE_MISMATCH
             case 2:
-                return 'type_mismatch';
+                return 'parse_error';      // ERROR_CONFIG_PARSE
             case 3:
-                return 'parse_error';
+                return 'flag_not_found';   // ERROR_FLAG_UNRECOGNIZED
             default:
                 return 'general';
         }

src/DDTrace/FeatureFlags/Provider.php

@@ -166,7 +166,9 @@ public function evaluate($flagKey, $variationType, $defaultValue, $targetingKey,
 
         // Error or no variant → return default
         if ($errorCode !== 0 || $result['variant'] === null) {
-            $evalResult = ['value' => $defaultValue, 'reason' => $reasonStr, 'variant' => null, 'allocation_key' => null,
+            // Per OpenFeature spec, any non-zero error code must use reason=ERROR
+            $reportReason = ($errorCode !== 0) ? 'ERROR' : $reasonStr;
+            $evalResult = ['value' => $defaultValue, 'reason' => $reportReason, 'variant' => null, 'allocation_key' => null,
                 'error_code' => $errorCode];
             FlagEvalMetrics::record($flagKey, $evalResult);
             return $evalResult;