Chat: fix welcome messages in chat view (#3155)

CLOSE #3085

The regex for allowed URIs in chat messages was updated to allow
additional custom URI schemes like 'command:cody.*'. This allows linking
directly to Cody guides and other custom VS Code commands from within
the chat.

Previously only the `_cody.vscode.open` command is allowed in the chat
view, which blocked the `Cody Commands` and `Getting Started Guide`
commands from working in the chat view.

This PR adds the `command:cody.` prefix to fix the aforementioned issue.


![image](https://github.com/sourcegraph/cody/assets/68532117/08b5118d-a1d9-4238-aa20-ff963fbde935)


## Test plan

<!-- Required. See
https://sourcegraph.com/docs/dev/background-information/testing_principles.
-->

Click on the `Cody Commands` and `Getting Started Guide` links in a new
chat to make sure they work:


https://github.com/sourcegraph/cody/assets/68532117/bf12648f-1f05-48da-bdeb-8a52dfb57eca

Author: abeatrix

lib/shared/src/chat/markdown.ts

@@ -11,7 +11,8 @@ import {
  *  - e.g. command:cody.welcome: VS Code command scheme exception we add to support directly linking to the welcome guide from within the chat.
  * {@link CODY_PASSTHROUGH_VSCODE_OPEN_COMMAND_ID}
  */
-const ALLOWED_URI_REGEXP = /^((https?|vscode):\/\/[^\s#$./?].\S*|command:(_cody.vscode.open\?.*))$/i
+const ALLOWED_URI_REGEXP =
+    /^((https?|vscode):\/\/[^\s#$./?].\S*|command:cody.*|command:(_cody.vscode.open\?.*))$/i
 
 const DOMPURIFY_CONFIG = {
     ALLOWED_TAGS: [

vscode/CHANGELOG.md

@@ -7,9 +7,12 @@ This is a log of all notable changes to Cody for VS Code. [Unreleased] changes a
 ### Added
 
 - Cody can now generate commit messages from your changes. Triggered through the input field in the VS Code "Source Control" sidebar. [pull/2306](https://github.com/sourcegraph/cody/pull/2306)
+- Chat: Displays warnings for large @-mentioned files during selection. [pull/3118](https://github.com/sourcegraph/cody/pull/3118)
 
 ### Fixed
 
+- Chat: Fixed an issue where the links in the welcome message for chat are unclickable. [pull/3155](https://github.com/sourcegraph/cody/pull/3155)
+
 ### Changed
 
 - Autocomplete: Removes the latency for cached completions. [https://github.com/sourcegraph/cody/pull/3138](https://github.com/sourcegraph/cody/pull/3138)