Validate the File constraint in the browser

[66Ton99]

Fixes formapro/JsFormValidatorBundle#116.

What was broken

@Assert\File had no JavaScript counterpart. src/Resources/public/js/constraints/
contained no File.js and index.js never imported one, so
SvarohJsFormValidator.parseConstraints() — which instantiates window[className] —
silently dropped the constraint. NotBlank on a file input worked, but maxSize,
mimeTypes, extensions and filenameMaxLength were only reported after a full
upload round trip. The maintainer confirmed on the issue that it was not implemented.

Two further reasons it could not just work:

• getSpecifiedElementTypeValue() read domNode.value for every input, which for a
  file input is only the fake path (C:\fakepath\avatar.png), never the files.
• Symfony\Component\Validator\Constraints\File::$maxSize is a protected property
  behind __get(). JsModelAbstract::phpValueToJs() exports constraints with
  get_object_vars(), which sees public properties only, so the limit never reached the
  browser at all. Every other option (binaryFormat, mimeTypes, extensions,
  filenameMaxLength, filenameCountUnit, all the messages) is public and was already
  exported generically.

What changed

• src/Resources/public/js/constraints/File.js (new). Checks what the File API can
  see, in Symfony's order and with Symfony's early returns: filename length →
  empty file → maxSize → extensions → mimeTypes. factorizeSizes() is ported from
  FileValidator (same k/M/Ki/Mi factors, same 2-decimal reduction, same
  bytes/kB/MB/KiB/MiB suffices), so maxSizeMessage renders
  {{ size }}/{{ limit }}/{{ suffix }} exactly as the server does.
  mimeTypesMessage, extensionsMessage, disallowEmptyMessage and the plural
  filenameTooLongMessage get their placeholders too. filenameCountUnit is honoured
  for bytes (UTF-8 length), codepoints and graphemes (Intl.Segmenter, falling
  back to code points).
• index.js / globals.test.js — register the constraint the way the others are.
• SvarohJsFormValidator.js — getSpecifiedElementTypeValue() now returns the
  files list of a file input via the new getFileListValue(). An empty selection is an
  empty array, so isValueEmty() keeps NotBlank working; without the File API it falls
  back to the previous string value.
• src/Factory/JsFormValidatorFactory.php — parseConstraints() exports a File
  constraint as its public options plus the normalized maxSize, keeping
  get_class($item) as the key so the JavaScript constructor name is unchanged.
• Docs — new src/Resources/doc/3_16.md plus a README index entry.

Deliberately not covered

• Image. Its own options (maxWidth, minHeight, maxRatio, …) need the file
  decoded into an Image object, which is asynchronous. The library has an async path
  (the ajax queue used by UniqueEntity), but it is built around a request/response
  round trip rather than a load event, and wiring image decoding into it is a separate
  piece of work. Rather than half-implement both, Image is left server-side only and
  documented as such; an Image constraint is skipped in the browser exactly as before.
• Mime type when the browser reports none. File.type is sniffed and can be empty or
  wrong. An empty type skips the mime check rather than rejecting the file, so the server
  keeps the last word.
• extensions narrowing the accepted mime types. That needs the symfony/mime media
  type database; only the extension itself is checked client-side, which can only be more
  permissive than the server, never less.
• filenameCharset, notFoundMessage, notReadableMessage and the upload*ErrorMessage
  family describe conditions that only exist server-side.

Tests

• File.test.js — 36 cases: the four size/suffix factorizations (including the two
  reduction loops), empty file, mime types with wildcards and with a string option,
  unsniffed type, extensions as a list and as a mime map, the extension and mime error
  reported together, the three filename count units and the Intl.Segmenter fallback,
  singular/plural filename message, multiple uploads, a real File object, and values
  that carry no file information. 100% statements / 96% branches on the new file.
• SvarohJsFormValidator.test.js — a case for the file input value: the files list, the
  empty list still being an empty value for NotBlank, and the fake-path fallback.
• JsFormValidatorFactoryTest::testFileConstraintExportsItsNormalizedMaxSize — asserts
  'maxSize':2097152 and 'binaryFormat':true reach the generated model for
  new Assert\File(maxSize: '2Mi', …). It errors on main because the constraint is
  still exported as an object without the option.

composer test 43 → 44 tests, 151 → 156 assertions. npx jest 34 → 35 suites,
347 → 385 tests. composer phpstan clean, git diff --check clean. No Cypress change:
the browser behaviour is fully covered by the Jest cases.

Note for the merger

src/Resources/doc/3_15.md is taken by two other open branches
(fix/issue-120-notblank-whitespace, fix/issue-148-constraint-property-path), so this
page is numbered 3_16.md / item 16 in the README index. It will need renumbering
depending on merge order.