fix: enable link click handling in VSelectableTextView (LUM-748)

[devin-ai-integration]

Summary

Links in chat messages were visually styled (tint color, underline, pointer cursor) but not clickable. The root cause: VSelectableTextView's NSTextView had linkTextAttributes configured but no delegate to handle click events.

This PR adds NSTextViewDelegate conformance to the existing Coordinator, implements textView(_:clickedOnLink:at:) to open URLs via NSWorkspace.shared.open(), and wires the delegate in makeNSView.

Review & Testing Checklist for Human

• Verify the project builds in Xcode (CI has no macOS build environment)
• Test clicking markdown links ([text](url)) in assistant chat messages — should open in default browser
• Test clicking autolinked bare URLs (e.g. https://example.com) in chat messages
• Test that text selection (click-drag, Cmd+A, Cmd+C) still works correctly with the delegate attached
• Verify no regressions in scroll performance with many messages containing links

Notes

• Coordinator now inherits from NSObject (required for the ObjC delegate protocol). This is standard for NSViewRepresentable coordinators that conform to AppKit delegate protocols.
• The delegate method handles both URL and String link values, covering all NSAttributedString link attribute types.
• A previous attempt at this fix existed on an unmerged branch (devin/1774970368-lum-635-selectable-text-view) using the same delegate approach, but it targeted an older file location before VSelectableTextView was moved to the design system.

Link to Devin session: https://app.devin.ai/sessions/c95781541a1246b49843bb46c01d006e
Requested by: @ashleeradka

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[devin-ai-integration]

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[devin-ai-integration]

Devin Review found 1 potential issue.

[devin-ai-integration]

🚩 Delegate link handler replicates default NSTextView behavior for non-editable text views

For a non-editable NSTextView (isEditable = false), the default behavior already opens links via NSWorkspace.shared.open() when no delegate intercepts the click. The new textView(_:clickedOnLink:at:) at line 249 does exactly what the default handler would do — open the URL in the default browser. This means the explicit delegate is currently a no-op from a behavioral standpoint.

This is likely intentional as a foundation for future customization (e.g., intercepting specific URL schemes, adding analytics, or routing internal links differently). However, if the only goal was to "enable" link clicking, the delegate wasn't needed — links already worked in the non-editable text view. Worth confirming the intent.

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[devin-ai-integration]

The claim that the default NSTextView behavior already opens links for non-editable text views is not accurate in this configuration. The user confirmed links are visually styled but completely inert (LUM-748 screenshot shows pointer cursor and underline but no click response).

Several factors can prevent the default link-opening fallback from firing in practice:

1. Explicit TextKit 1 stack — this NSTextView is created with a manually constructed TextKit 1 stack (NSTextStorage → NSLayoutManager → NSTextContainer), not the default initializer. The default clicked(onLink:at:) fallback behavior may not be wired identically in this path.
2. isSelectable = true + isEditable = false — in this mode, mouse events are primarily routed through the selection machinery. Link click detection depends on the text view correctly distinguishing a click-on-link from a selection gesture, which can be unreliable without an explicit delegate.
3. Previous fix attempt — an earlier branch (devin/1774970368-lum-635-selectable-text-view) independently reached the same conclusion and used the delegate approach.

The explicit delegate is the Apple-recommended pattern for handling link clicks (NSTextViewDelegate.textView(_:clickedOnLink:at:)). It provides reliable, deterministic behavior regardless of the TextKit stack configuration, and gives us a hook for future customization (e.g., routing internal links, analytics).