refactor(formatter): use if let to fix unnecessary_unwrap warning

[sasezaki]

📌 What Does This PR Do?

The printer logic was using is_some() followed by unwrap(), which triggered a clippy::unnecessary_unwrap warning.

🔍 Context & Motivation

just check raises below error.

error: called `unwrap` on `group.expanded_states` after checking its variant with `is_some`
   --> crates/formatter/src/internal/printer/mod.rs:498:51
    |
497 |                     if group.expanded_states.is_some() && group_mode.is_break() {
    |                        ------------------------------- the check is happening here
498 |                         if let Some(last_state) = group.expanded_states.as_ref().unwrap().last() {
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: try using `if let` or `match`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_unwrap
    = note: `-D clippy::unnecessary-unwrap` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::unnecessary_unwrap)]`

error: could not compile `mago-formatter` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `mago-formatter` (lib test) due to 1 previous error

🛠️ Summary of Changes

• Refactor: refactors the code to use if let for safe and idiomatic pattern matching on the Option type.

📂 Affected Areas

• Linter
• Formatter
• CLI
• Composer Plugin
• Dependencies
• Documentation
• Other (please specify):

🔗 Related Issues or PRs

📝 Notes for Reviewers

With this PR, just check would be passed.

2025-12-20T04:40:45.904904Z DEBUG main mago::config: Configuration specifies a stack size of 12582912 bytes.
2025-12-20T04:40:46.675013Z  INFO main mago::service: No issues found.
cargo +nightly fmt --all -- --check --unstable-features
cargo +nightly clippy --workspace --all-targets --all-features -- -D warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.21s
cargo +nightly check --workspace --locked
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.16s