accordion: Preserve disabled state of individual items

[violetpurpleish]

Description

An enabled Accordion overwrote each item's own disabled flag, making .disabled(true) items interactive. Combine the group and item flags so either one can disable the item. Add a regression covering enabled siblings and disabling/re-enabling the group, plus a disabled item in the Accordion story.

Public API

gpui-component (existing signatures, corrected behavior):

• Accordion::disabled(self, disabled: bool) -> Self: disables the group while preserving independently disabled children.
• AccordionItem::disabled(self, disabled: bool) -> Self: an item's disabled flag remains effective inside an enabled group.

Breaking Changes

Code relying on the old accidental enabling behavior must explicitly enable the item. Builder signatures are unchanged.

 Accordion::new("settings").item(|item| item.title("Advanced").disabled(true))
-// Clicking Advanced can expand it.
+// Clicking Advanced leaves it closed.

How to Test

• cargo test -p gpui-kit --features test-support --test disclosure --locked: 5 passed. The new regression failed on main before the fix.
• Native macOS Story, launched with ./script/run-story-macos Accordion: click Disabled item (stays closed), click Can it hold any content? (expands), toggle Options > Disabled (all triggers ignore clicks), re-enable the group (Disabled item stays closed).
• Used Computer accessibility element actions. The native tree exposes unlabeled Accordion buttons without expanded values, so screenshots identified the buttons and verified panel visibility.
• Full Component/Kit suite, formatting, Clippy, and Metal rendering checks also passed in a temporary checkout combining the four independent fixes. Windows/Linux execution remains for CI.

AI-assisted implementation with Codex; diff reviewed and regression/native checks run.

[huacnlee]

Thanks