fix: make IssueKind public and export it in __all__

[kredd2506]

Description

Rename _IssueKind → IssueKind (drop the leading underscore) and add it to __all__ in prompt_validation.py.

ValidationIssue.issue_kind is a public field typed as _IssueKind, a private enum not included in __all__. This forces users to import a private symbol to filter individual issues:

# Before (accessing private symbol)
from langextract.prompt_validation import _IssueKind
failed = [i for i in report.issues if i.issue_kind == _IssueKind.FAILED]

# After (clean public API)
from langextract.prompt_validation import IssueKind
failed = [i for i in report.issues if i.issue_kind == IssueKind.FAILED]

Fixes #389

Bug fix

How Has This Been Tested?

$ pytest tests/prompt_validation_test.py -q
15 passed in 1.18s

$ pytest tests/ -q --ignore=tests/inference_test.py
370 passed, 76 warnings, 29 subtests passed

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page, and I either signed the Google Individual CLA or am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed Google's Python Style Guide and ran pylint over the affected code.

[github-actions]

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