[configuration] Fix unclear error messages for line-length values exceeding u16::MAX

[danparizher]

Summary

Fixed unclear error messages when line-length configuration values exceed u16::MAX (65535). Previously, values > 65535 would produce a cryptic TOML parsing error "expected u16" instead of a clear validation message. Now all invalid values produce consistent, user-friendly error messages.

Fixes #21328

Problem Analysis

The LineLength type's Deserialize implementation attempted to deserialize directly as u16. When TOML contained values exceeding u16::MAX (65535), the TOML deserializer would fail before reaching the validation logic, resulting in unclear error messages:

• Values > 65535: "invalid value: integer 65536, expected u16" (unclear)
• Values 321-65535: "line-length must be between 1 and 320 (got 500)" (clear)

This inconsistency violated the TOML spec's requirement to accept arbitrary 64-bit signed integers and handle validation errors gracefully.

Approach

Modified the Deserialize implementation for LineLength in crates/ruff_linter/src/line_width.rs to:

1. Deserialize as u64 first: Accept any valid TOML integer value
2. Validate u16 bounds: Check if the value exceeds u16::MAX before conversion
3. Provide consistent errors: All out-of-range values now produce the same clear error message format: "line-length must be between 1 and 320 (got {value})"

Added test cases covering:

• Boundary case: line-length = 65535 (at u16::MAX)
• Exceeds u16::MAX: line-length = 65536
• Far exceeds u16::MAX: line-length = 99_999

All tests verify that the error messages are clear and consistent regardless of whether the value exceeds u16::MAX or just exceeds the valid range (1-320).

[MichaReiser]

Thank you.

For us reviewers, it's very helpful to get feedback when suggestions don't work (I obviously didn't realize that Rust won't like and_then over different error types) and/or resolve suggestions. It helps us understand which comments were addressed and, if not, why not (without having to clone the PR and applying the changes manually).