fix(chat): widen selection code fence so embedded backticks can't truncate it

Cursor: resolveFileSelectionResource wrapped the selected passage in a fixed ```
fence, so a selection that itself contained a fenced code block closed the outer
fence early and the agent received a truncated snippet. The fence is now one
backtick longer than the longest backtick run in the content (floored at three),
matching CommonMark's close rule.

Author: mzxchandra

apps/sim/lib/copilot/chat/process-contents.test.ts

@@ -355,6 +355,35 @@ describe('processContextsServer - file_selection contexts', () => {
 
     expect(result).toEqual([])
   })
+
+  it('widens the code fence so an embedded ``` block cannot close it early', async () => {
+    getWorkspaceFile.mockResolvedValue({ name: 'readme.md', folderPath: null })
+
+    const snippet = 'before\n```ts\nconst x = 1\n```\nafter'
+    const result = await processContextsServer(
+      [
+        {
+          kind: 'file_selection',
+          fileId: 'file-1',
+          label: 'readme.md:1-5',
+          text: snippet,
+          startLine: 1,
+          endLine: 5,
+        } as ChatContext,
+      ],
+      'user-1',
+      'explain',
+      'ws-1'
+    )
+
+    const [ctx] = result
+    // Outer fence must be longer than the embedded ``` run, and the full snippet
+    // (including its inner fence) must survive intact.
+    expect(ctx.content).toContain('````')
+    expect(ctx.content).toContain(snippet)
+    expect(ctx.content.startsWith('Selected passage')).toBe(true)
+    expect(ctx.content.endsWith('````')).toBe(true)
+  })
 })
 
 describe('processContextsServer - table_selection contexts', () => {

apps/sim/lib/copilot/chat/process-contents.ts

@@ -878,6 +878,21 @@ async function resolveFileResource(
   }
 }
 
+/**
+ * Picks a backtick fence long enough to wrap `content` without an embedded
+ * backtick run closing it early. Per CommonMark, a fenced block ends only on a
+ * run of at least as many backticks as the opener, so the fence is one longer
+ * than the longest run inside the content, floored at the standard three. Keeps
+ * a selection that itself contains a ``` code block from truncating the snippet.
+ */
+function codeFenceFor(content: string): string {
+  let longest = 0
+  for (const match of content.matchAll(/`+/g)) {
+    longest = Math.max(longest, match[0].length)
+  }
+  return '`'.repeat(Math.max(3, longest + 1))
+}
+
 /**
  * Resolves a highlighted passage from a file into an inline, citable snippet.
  * The selected text travels with the request (it is the user's own content), so
@@ -902,7 +917,8 @@ async function resolveFileSelectionResource(
       : startLine
         ? ` (line ${startLine})`
         : ''
-  const content = `Selected passage from ${record.name}${lineRange}:\n\n\`\`\`\n${snippet}\n\`\`\``
+  const fence = codeFenceFor(snippet)
+  const content = `Selected passage from ${record.name}${lineRange}:\n\n${fence}\n${snippet}\n${fence}`
   return {
     type: 'file_selection',
     tag: label ? `@${label}` : '@',