Pin cocoa-foundation to avoid deprecation warnings

[Vanuan]

Objective

Prevent build failures on macOS targets caused by deprecation warnings in cocoa-foundation v0.2.1 when compiling with -D warnings / #![deny(warnings)].

When Cargo resolves dependencies for crates depending on cocoa (such as gpui_apple), cocoa v0.26.0 pulls the latest compatible patch version of cocoa-foundation (0.2.1). Because upstream marked cocoa-foundation 0.2.1 as deprecated in favor of objc2, compiler deprecation warnings are emitted, causing hard build failures in strict warning environments.

Solution

• Pinned cocoa-foundation to exact version =0.2.0 in the workspace manifest (Cargo.toml) and referenced it where necessary to constrain transitive resolution.

Testing

• Verified that cargo update -p gpui_apple doesn't update cocoa-foundation from 0.2.0 to 0.2.1.
• Verified with cargo tree -i cocoa-foundation@0.2.0 that all transitive instances of cocoa-foundation resolve strictly to 0.2.0.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards ([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist) and [icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md) guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

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Release Notes:

• N/A