int(webp, jpeg-xl): Added check_open for WebP and JPEG XL readers

[hkgulka]

Description

This PR addresses the feature request in issue #3974

Continuing from previous work done on this feature request, I have added check_open calls to two new format readers, using file format specs found online:

• WebP: https://developers.google.com/speed/webp/faq#what_is_the_maximum_size_a_webp_image_can_be
• JPEG XL: https://jpegxl.info/resources/battle-of-codecs.html

I also cross-referenced this info with the corresponding ImageOutput::check_open calls for sanity, and they seemed to align.

Tests

Checklist:

• I have read the guidelines on contributions and code review procedures.
• I have read the Policy on AI Coding Assistants
  and if I used AI coding assistants, I have an Assisted-by: TOOL / MODEL
  line in the pull request description above.
• I have updated the documentation if my PR adds features or changes
  behavior.
• I am sure that this PR's changes are tested in the testsuite.
• I have run and passed the testsuite in CI before submitting the
  PR, by pushing the changes to my fork and seeing that the automated CI
  passed there. (Exceptions: If most tests pass and you can't figure out why
  the remaining ones fail, it's ok to submit the PR and ask for help. Or if
  any failures seem entirely unrelated to your change; sometimes things break
  on the GitHub runners.)
• My code follows the prevailing code style of this project and I
  fixed any problems reported by the clang-format CI test.
• If I added or modified a public C++ API call, I have also amended the
  corresponding Python bindings. If altering ImageBufAlgo functions, I also
  exposed the new functionality as oiiotool options.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: hkgulka / name: Hannah Gulka (d238709)

[lgritz]

The failure for the "VFX2023" test is unrelated to your PR and is being fixed separately. If everything else is ok, we will not hold up the merge just because of that.

[lgritz]

LGTM! Thanks for the patch