fix:add validation and comments

[Dailiduzhou]

Summary

Added detailed comments to the Frame::check function's bulk string (b'$') case in src/frame.rs to explain the RESP (Redis Serialization Protocol) format and validation logic.
Added validation for special cases, aligning with Frame::parse.

Changes

File: src/frame.rs
Added comments and validation:

               // Bulk strings in the RESP protocol.
               //
               // Format: $<length>\r\n<data>\r\n
               //
               // Special case: $-1\r\n represents a Null value.
               // Validates that the frame conforms to RESP protocol.
               if b'-' == peek_u8(src)? {
                    let line = get_line(src)?;
                    if line != b"-1" {
                        return Err("protocol error; invalid frame format".into());
                    }
                    Ok(())
                } else {
                    // Read the bulk string
                    let len: usize = get_decimal(src)?.try_into()?;

                    // skip that number of bytes + 2 (\r\n).
                    skip(src, len + 2)
                }

Rationale

• The bulk string case previously had minimal documentation
• Clarifies the expected RESP protocol format for new contributors
• Documents the special Null value case ($-1\r\n)
• Improves code maintainability by making the protocol validation logic explicit

[Darksonn]

Can you add a test case where invalid data is sent here, and assert that it is rejected?