Fix LiteralUTF8Char lowering for non-ASCII UTF-8 chars

[tmdeveloper007]

Pull Request description

This PR fixes a Unicode lowering bug in LiteralUTF8Char.

When lowering astx.LiteralUTF8Char, the code correctly computed the UTF-8 byte length, but initialized the backing global constant using ASCII encoding. That caused translation to fail for valid non-ASCII characters such as é with a UnicodeEncodeError.

Changes made:

• use UTF-8 bytes when initializing LiteralUTF8Char storage
• add a regression test covering a multibyte UTF-8 char literal

Addresses #208

How to test these changes

• run pytest tests/test_string.py -q -k "utf8_char_non_ascii_translate"
• confirm the test passes
• optionally verify that lowering a module containing astx.LiteralUTF8Char("é") no longer raises UnicodeEncodeError

Pull Request checklists

This PR is a:

• bug-fix
• new feature
• maintenance

About this PR:

• it includes tests.
• the tests are executed on CI.
• the tests generate log file(s) (path).
• pre-commit hooks were executed locally.
• this PR requires a project documentation update.

Author's checklist:

• I have reviewed the changes and it contains no misspelling.
• The code is well commented, especially in the parts that contain more
  complexity.
• New and old tests passed locally.

Additional information

• Validation run locally:
  • pytest tests/test_string.py -q -k "utf8_char_non_ascii_translate"
  • ruff check src/irx/builders/llvmliteir.py tests/test_string.py
• I kept the change minimal and limited it to the UTF-8 char literal lowering path and one regression test.

Reviewer's checklist

Copy and paste this template for your review's note:

## Reviewer's Checklist

- [ ] I managed to reproduce the problem locally from the `main` branch
- [ ] I managed to test the new changes locally
- [ ] I confirm that the issues mentioned were fixed/resolved .

[yuvimittal]

@tmdeveloper007 , the tests currently only testing that LLVM IR generation doesn't crash, not that the UTF-8 character lowering is correct.

[yuvimittal]

what is the use of tmp here?