Validate NULL bulk strings have expected format

[Dailiduzhou]

Context

This PR supersedes #155 with two improvements:

1. Includes a regression test case for the validation logic
2. Created from a dedicated fix branch (fix/add-validation) instead of master, following proper contribution workflow

Apologies for the earlier confusion with the branch source.

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

Source code

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)
                }

Test case

File:test/frame_validation.rs

use mini_redis::frame::{Error, Frame};
use std::io::Cursor;

#[test]
fn check_rejects_invalid_negative_bulk_length() {
    // Valid null bulk string in RESP.
    let mut ok = Cursor::new(&b"$-1\r\n"[..]);
    assert!(Frame::check(&mut ok).is_ok());

    // Invalid negative bulk length.
    let mut bad = Cursor::new(&b"$-2\r\n"[..]);
    let err = Frame::check(&mut bad).unwrap_err();

    match err {
        Error::Other(e) => {
            assert_eq!(e.to_string(), "protocol error; invalid frame format");
        }
        Error::Incomplete => panic!("expected protocol error, got incomplete"),
    }
}

All tests pass locally

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

[Dailiduzhou]

Apologies for the earlier confusion with the branch source. @Darksonn

[Darksonn]

Thanks.