[mojo] Update the codebase to Mojo v25.5 nightly

[forfudan]

This pull request updates the codebase to Mojo v25.5 (nightly).

Dependency and Configuration Updates:

• Updated the pixi.toml file to increment the version from 0.4.1 to 0.5.0, switch the max dependency to a more flexible version constraint (>25.4), and include the max-nightly channel in the channels list.

Enhancements to BigDecimal:

• Added new traits (AnyType, Copyable, Movable, Representable) to the BigDecimal struct to enhance compatibility and functionality.
• Updated various methods in BigDecimal to use Self instead of BigDecimal, improving type consistency and reducing redundancy. This includes methods like extend_precision, normalize, and comparison operators (__gt__, __lt__, etc.).

Enhancements to BigInt:

• Added new traits (AnyType, Comparable, Copyable, Movable) to the BigInt struct, aligning it with the enhancements made to BigDecimal.
• Updated initialization methods (from_list, __init__) to use var for input parameters, reflecting a safer and more flexible approach to handling ownership and mutability.

Optimizations in BigUInt Arithmetic:

• Replaced data with _data for internal word storage in SIMD-based arithmetic methods (e.g., add_slices_simd, subtract_simd), improving encapsulation and clarity.
• Removed unnecessary imports (time, testing) across multiple files to streamline the codebase.

Other Improvements:

• Changed the @value decorator to @fieldwise_init in the Rational struct to reflect a more appropriate initialization strategy.