Fix PLW0129 for conditional expressions - #30434

Summary

Fixes #30432.

Extend PLW0129 (assert-on-string-literal) to detect string literals in both branches of a top-level conditional expression used in an assert statement.

For example, assert value if condition else "fallback" should be flagged because one branch always evaluates to a non-empty string, making the assertion always pass.

The implementation reuses the existing string-literal diagnostic logic and checks only the direct branches of the conditional expression. Comparison-wrapped conditionals are left unchanged.

Added regression tests covering:

• Non-empty strings in both branches.
• Empty strings in both branches.
• Conditional expressions inside comparisons, which should not be flagged.

Notes to reviewers

The focused test, formatting checks, prek checks, and git diff --check all pass.