Fix useless string cast on already-string-typed map keys in normalize()

MapType::createNormalizationLoopOutputAssignement() unconditionally cast
the loop key to string, but normalization always iterates a getter whose
declared return type is this MapType's own array<string, ...> contract
(see getDocTypeHint()) -- the key is already provably string, so PHPStan
correctly flags the cast as dead code (cast.useless).

Denormalization's createLoopOutputAssignement() is untouched: it processes
raw mixed-typed external data where the key's type isn't statically known,
so the cast there is legitimately defensive.

Surfaced by Zoho CRM's record.json spec (a project consuming this fork via
require-dev) generating a normalizer for a free-form additionalProperties
map for the first time -- Desk's specs never exercised this MapType
normalization path.

Author: LTSCommerce

src/Component/GeneratorCore/Guesser/Guess/MapType.php

@@ -71,6 +71,12 @@ protected function createNormalizationLoopOutputAssignement(Expr $valuesVar, ?Ex
             throw new LogicException('MapType requires a loop key expression');
         }
 
-        return new Expr\ArrayDimFetch($valuesVar, new Expr\Cast\String_($loopKeyVar));
+        // Unlike createLoopOutputAssignement() (denormalization, where the source is
+        // raw mixed-typed external data and the key's runtime type isn't statically
+        // known), normalization always iterates a getter whose declared return type
+        // is this very MapType's own array<string, ...> (see getDocTypeHint() above)
+        // -- so $loopKeyVar is already provably `string` and casting it is dead code
+        // PHPStan correctly flags as cast.useless.
+        return new Expr\ArrayDimFetch($valuesVar, $loopKeyVar);
     }
 }