Fix u32 overflow in the Rust duration parser

[Sharadhi-v98]

1. parse_duration_number accumulates each unit's digits into a u32 with a plain multiply and add, and the hours, minutes and days accumulators (plus the weeks-fraction carry onto days) are added to unchecked as well.
2. Release wheels set overflow-checks = false, so the value wraps silently: PT4294967296S parses to an empty Duration, P4294967297D to one day and PT4294967295H1H to zero hours. A debug build panics on the same input and the PanicException escapes the parser's ValueError handling.

Made the digit accumulation and those additions checked, so an oversized value fails with a ParseError like the existing timezone offset bound.

• Added tests for changed code.
• Updated documentation for changed code.