feat(serialization): add two-decimal precision serialization for floa…

[fslongjin]

feat(serialization): add two-decimal precision serialization for float fields

Added a new serialize_f32_two_decimals helper function to serialize Option fields with exactly two decimal places. Applied this serialization to all float fields in ChatBody (temperature, top_p, n, presence_penalty, frequency_penalty) to ensure consistent numeric formatting in API requests.

Also improved error handling in requests.rs by:

1. Adding fallback for JSON parsing errors
2. Enhancing error messages with status codes
3. Making error messages more descriptive