fix(menu): return error instead of panicking on unexpected ItemKind

[krishpranav]

fix(menu): return error instead of panicking on unexpected ItemKind

Problem

Menu plugin commands take kind straight from the IPC payload and dispatch on it through the do_menu_item! macro, whose accepted kind set never includes the root Menu variant (and some call sites narrow it further). Anything outside that set fell through to _ => unreachable!() and took the whole process down.

Reachable from plain JS — the TS types don't stop it, and raw IPC ignores them:

await Menu.new({ items: [await Menu.new()] })

Fix

• Changed the do_menu_item! fallback arm from unreachable!() to a recoverable error (anyhow::anyhow!("unexpected menu item kind").into()), matching the hand-written _ => return Err(...) arms already in menu/plugin.rs. One macro change covers all nine call sites.
• Added a regression test that drives the affected commands with mismatched kinds and asserts they return Err instead of panicking.
• Added .changes/fix-menu-plugin-panic.md with valid covector front matter ("tauri": patch:bug, "tauri-macros": patch:bug).

Testing

cargo fmt --check
cargo clippy -p tauri -p tauri-macros --all-features -- -D warnings
cargo test -p tauri --lib menu::plugin::tests

The regression test panics on dev and passes with this change.

Before (fallback arm still unreachable!()):

test menu::plugin::tests::unexpected_item_kind_errors_instead_of_panicking ... FAILED

thread '...' panicked at crates/tauri/src/menu/plugin.rs:751:3:
internal error: entered unreachable code

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 57 filtered out

After (this PR):

test menu::plugin::tests::unexpected_item_kind_errors_instead_of_panicking ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 57 filtered out; finished in 0.01s

Visual Verification

App survives the repro (examples/api)

Ran the exact repro from the issue (Menu.new({ items: [await Menu.new()] })). On dev the process aborts; with this change the call rejects with unexpected menu item kind and the window stays alive.

Fixes #15686

[Legend-Master]

This is a breaking change, we'll need to either add a new macro or add an optional parameter to this macro