fix: drop @internal annotation from InputField

[oojacoboo]

Closes #761.

Problem

InputField is marked @internal, but userland cannot avoid referencing it: the public InputFieldMiddlewareInterface::process() method signature returns InputField|null, so any middleware implementation must import and return InputField directly.

public function process(
    InputFieldDescriptor $inputFieldDescriptor,
    InputFieldHandlerInterface $inputFieldHandler,
): InputField|null;

The @internal marker contradicts that public-API contract and causes static analysis tooling to flag legitimate middleware implementations.

Fix

Drop the @internal line from the InputField class docblock. The class is already final, which keeps its inheritance contract locked down; removing @internal just acknowledges that the type is part of the public surface reachable through the middleware interface.

Note: the sibling QueryField class retains @internal because FieldMiddlewareInterface::process() returns the parent FieldDefinition, not QueryField — userland middleware never needs to name QueryField directly.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.73%. Comparing base (53f9d49) to head (9bef07a).
⚠️ Report is 145 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #795      +/-   ##
============================================
- Coverage     95.72%   94.73%   -0.99%     
- Complexity     1773     1832      +59     
============================================
  Files           154      172      +18     
  Lines          4586     4808     +222     
============================================
+ Hits           4390     4555     +165     
- Misses          196      253      +57     

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