fix(http): recursively map nested uploaded files

[osbre]

PSR-7 exposes array-style file inputs, such as files[] or files[avatar], as nested arrays. The mapper previously assumed a single-level array with a callback typed to UploadedFileInterface, so passing a nested array threw TypeError.

Uploaded files are now mapped recursively to preserve this nested structure, and the $files docblocks on Request and IsRequest have been widened to array<array-key, Upload|array>.

Runtime behaviour for standard file uploads is unchanged, though static analysis tools may report new notices in code that assumes Upload[].

[github-actions]

Benchmark Results

Comparison of fix/nested-request-files against 3.x (7e5cff65349956e14578e826a95d1f3ce6953761).

Open to see the benchmark results

No benchmark changes above ±5%.

_{Generated by phpbench against commit d698ba0}

[xHeaven]

I'd prefer a test that actually exercises the reported behavior, other than that, LGTM.
Something like:

#[Test]
public function nested_uploaded_files_are_mapped_to_uploads(): void
{
    $request = $this->createServerRequest('POST')->withUploadedFiles([
        // ...
    ]);

    $mappedRequest = $this->mapper->map(
        from: $request,
        to: GenericRequest::class,
    );

    // asserts
}

[osbre]

Thanks @xHeaven - I updated the test