Support extension-owned handling of links detected in chat

An extension should be able to:
- Declare supported link patterns.
- Customize permitted display or hover metadata
- Intercept matching clicks before normal navigation.
- Route clicks to an extension-owned action or Canvas with the parsed URL and structured parameters.
- Explicitly allow default navigation when appropriate.

Scenario: I want to integrate with our ticket managing system, automatically hyperlinking the (Jira) ticket #s. Hovering would show a nice card (if I had the ability to do this), and clicking would open up a canvas w/the selected ticket. Currently, I can hyperlink the tickets and I can get them to open in teh chat, but it also opens up an external browser window (not what I want).

Start by tracing the existing chat hyperlink behavior described in the issue, including how matching links open in chat and how the external browser is triggered. Review the extension link-handling and Canvas entry points if available. Done means extensions can declare patterns, control metadata, intercept clicks, route parsed URLs and parameters to an action or Canvas, and explicitly permit default navigation.

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