Fix checkbox checkedness not updating when checked attribute is cleared

[nicoburns]

Summary

Fixes DioxusLabs/dioxus#5282 (signal-driven checked on <input type="checkbox"> updates once then sticks).

Root cause: dioxus-native-dom translates a falsy checked value into DocumentMutator::clear_attribute, but clear_attribute only removed the attribute from element.attrs — it never updated the element's checkedness stored in SpecialElementData::CheckboxInput. So false -> true worked (via set_attribute → set_input_checked_state), but true -> false left the checkbox visually checked.

Fix in clear_attribute:

// after removing the attr, before the `!had_attr` early return
if (tag, attr) == tag_and_attr!("input", "checked") {
    if let Some(checked) = element.checkbox_input_checked_mut() {
        *checked = false; // + mark mutations_occurred
    }
}

This runs even when the attribute wasn't present, since checkedness can also be set by a user click without the attribute ever existing.

Adds regression tests in tests/blitz-tests/tests/checkbox_checked.rs driving a checkbox via a Dioxus prop through true/false transitions (both failed before the fix).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/9d81d7c5e6f24e0e92d3da00b4f798ab
Requested by: @nicoburns

WPT results

No changes in test results compared to main.

_{Generated by the WPT workflow.}

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