[Clippy] Fix various clippy warnings

[sorafujitani]

Summary

Address multiple clippy warnings to improve code quality and idiomatic Rust usage.

Warnings to Fix

1. unused_self warnings

File: ext/rfmt/src/parser/prism_adapter.rs:17, 80

fn parse_json(&self, json: &str) -> ...  // &self unused
fn convert_comment(&self, comment: &PrismComment) -> ...  // &self unused

Fix: Convert to associated functions:

fn parse_json(json: &str) -> ...
fn convert_comment(comment: &PrismComment) -> ...

2. match_same_arms warning

File: ext/rfmt/src/parser/prism_adapter.rs:82-91

"line" => CommentType::Line,
"block" => CommentType::Block,
_ => CommentType::Line,  // Same as "line" arm

Fix: Combine arms:

"block" => CommentType::Block,
_ => CommentType::Line,

3. doc_markdown warnings

Files: ext/rfmt/src/error/mod.rs:32, ext/rfmt/src/parser/prism_adapter.rs:16,29,79

/// Convert RfmtError to Magnus Error for Ruby interop

Fix: Add backticks around type names:

/// Convert `RfmtError` to Magnus `Error` for Ruby interop

4. struct_excessive_bools warning

File: ext/rfmt/src/ast/mod.rs:423

pub struct FormattingInfo {
    pub needs_blank_line_before: bool,
    pub needs_blank_line_after: bool,
    pub preserve_newlines: bool,
    pub multiline: bool,
}

Fix (optional): Use bitflags:

use bitflags::bitflags;

bitflags! {
    pub struct FormattingFlags: u8 {
        const NEEDS_BLANK_LINE_BEFORE = 0b0001;
        const NEEDS_BLANK_LINE_AFTER = 0b0010;
        const PRESERVE_NEWLINES = 0b0100;
        const MULTILINE = 0b1000;
    }
}

Files to Modify

• ext/rfmt/src/parser/prism_adapter.rs
• ext/rfmt/src/error/mod.rs
• ext/rfmt/src/ast/mod.rs (optional bitflags change)

Acceptance Criteria

• cargo clippy reports no warnings
• All existing tests pass
• No functional changes to behavior