fix: compute padding correctly for MAX_LENGTH (0xffff) content

[iadev09]

The previous padding calculation used signed negation via i16, which could overflow for large content lengths. Switch to wrapping unsigned arithmetic so padding is computed correctly across the full u16 range, including 0xffff (MAX_LENGTH)

This can be reproduced in debug builds with large content lengths, resulting in:

attempt to negate with overflow
/fastcgi-client-0.11.0/src/meta.rs:168:30
stack backtrace:
0: __rustc::rust_begin_unwind

[Copilot]

Pull request overview

This PR fixes FastCGI Header padding computation for large content_length values by avoiding signed negation overflow and using wrapping unsigned arithmetic, ensuring correct padding across the full u16 range up to MAX_LENGTH (0xffff).

Changes:

• Replace i16-based negation with u16::wrapping_sub to compute padding_length safely for all content_length values.

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