fix: prevent panic when formatting complex generics

[avrabe]

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.

[ding-young]

Hi, I just came across your PR. Would it be alright if I try it out to see if https://github.com/rust-lang/rustfmt/pull/6396 fixes the unwrap error in that test case? (This seems orthogonal to the mentioned PR though, so just to verify whether the issue is duplicate or not)

[avrabe]

@ding-young Sure, would be good to see. https://github.com/pulseengine/wrt is where my problem originate from. My patch makes this repository format again. Otherwise it actually stripped away code.