Fixes #6571

Problem

rustfmt panics with Option::unwrap() on a None value when formatting enums with very long generic type constraints. This occurs when the generic constraints exceed the available width budget (~80-100 characters), causing rewrite_generics to fail and format_generics to return None, which is then unwrapped.

Solution

This PR implements a three-tier fallback strategy in format_generics:

1. First attempt: Try formatting with normal width constraint
2. Second attempt: If that fails, retry with infinite width using shape.infinite_width()
3. Fallback: If even that fails, preserve the original source formatting

Changes

• Modified format_generics function in src/items.rs to handle width budget exhaustion gracefully
• Added regression test case issue-6571.rs to prevent future regressions

Testing

• All existing tests pass
• New regression test verifies the fix works for complex generic constraints
• Tested on the original failing codebase that triggered the issue

This fix ensures rustfmt can process files with complex generic constraints without crashing, while maintaining proper formatting where possible.