text: Allow applications to handle TextView link clicks (#2621)

## Description

Adds an optional `TextView::on_link_click` callback carrying the
resolved URL, mouse button, and keyboard modifiers. Applications can
route rendered Markdown or HTML links internally and distinguish
ordinary, modified, middle, and right clicks.

When a handler is installed, the click is consumed and the application
owns routing. Without a handler, links retain the existing
`App::open_url` behavior. The implementation covers inline text links
and linked images. Linked images register both primary and auxiliary
click listeners so middle and right clicks reach the callback; right
clicks without a handler retain the prior fallback behavior.

Closes #2620.

## Testing

Exact candidate `11bb95f2` on Rust 1.97.1:

- `cargo test -p gpui-component text_view --lib`: 12 passed
- `cargo test -p gpui-component`: 347 passed
- `cargo clippy -p gpui-component --all-targets -- -D warnings`
- rendered linked-image regression proves left, middle, and right
delivery to the custom handler and proves that the default URL opener is
not called

The current public head is green on Windows, macOS, and Linux.

## AI Assistance

The implementation and tests were AI-assisted by Friday. Chris Hynes
defined the API requirements, directed the auxiliary-click fixes, and
owns the final review.

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Co-authored-by: Friday <260232009+buildfriday@users.noreply.github.com>
Co-authored-by: Jason Lee <huacnlee@gmail.com>