test: add contract tests for @-mention extractor boundary

Pins the contract between `extractAtMentionedFiles` and
`extractMcpResourceMentions` so the MCP regex can't silently swallow
quoted file-path mentions.

These tests fail on current HEAD — 3 of 11 cases expose the regression
pointed out in the review on #382: `extractMcpResourceMentions`'s
trailing `\b` backtracks past the closing `"` of a quoted mention and
produces a ghost match for `@"C:\Users\..."`, `@C:\Users\...`, and
`@"/tmp/weird:name.txt"`. The remaining 8 cases lock in the behaviour
that must not change (legitimate `server:resource` mentions and plain
file-path mentions).

Committed failing on purpose as the first half of a test-then-fix
pair; the regex fix follows in a subsequent commit.

src/utils/attachments.extractors.test.ts

@@ -0,0 +1,85 @@
+import { describe, expect, test } from 'bun:test'
+import {
+  extractAtMentionedFiles,
+  extractMcpResourceMentions,
+} from './attachments.js'
+
+// Contract tests for the two @-mention extractors.
+//
+// Scope: the narrow contract between `extractAtMentionedFiles` and
+// `extractMcpResourceMentions` where both are called on the same input
+// and must not both claim the same token. The motivating bug is that
+// `extractMcpResourceMentions`'s `\b` anchor lets it backtrack over the
+// closing quote of a quoted file mention, producing a ghost match for
+// `@"C:\Users\..."`. These tests pin the boundary so any regression in
+// the MCP regex is caught immediately.
+describe('extractor contract', () => {
+  describe('extractMcpResourceMentions must return empty for', () => {
+    const cases: Array<[string, string]> = [
+      // Primary bug: the quoted form that PromptInput emits for Windows
+      // paths today. `\b` backtracks past the trailing `"` and produces
+      // a ghost MCP match on current HEAD.
+      ['a quoted Windows drive-letter path', '@"C:\\Users\\me\\file.txt"'],
+      // Even if the quote layer were stripped, a bare drive letter
+      // followed by a path separator is never an MCP resource.
+      ['an unquoted Windows drive-letter path', '@C:\\Users\\me\\file.txt'],
+      // Sanity: quoted POSIX paths with no `:` at all never matched the
+      // MCP regex and must keep not matching after the fix.
+      ['a quoted POSIX path with a space', '@"/Users/foo/my file.ts"'],
+      ['an unquoted POSIX path', '@/Users/foo/bar.ts'],
+      // Quoted POSIX path that embeds a `:` in the filename — the quote
+      // layer must shield it from MCP matching, same as the Windows case.
+      ['a quoted POSIX path with a colon in the name', '@"/tmp/weird:name.txt"'],
+    ]
+    test.each(cases)('%s', (_label, input) => {
+      expect(extractMcpResourceMentions(input)).toEqual([])
+    })
+  })
+
+  describe('extractMcpResourceMentions still matches legitimate MCP mentions', () => {
+    // Regression guard for the fix. If someone tightens the MCP regex
+    // too aggressively, these break and the intent is clear.
+    const cases: Array<[string, string, string[]]> = [
+      [
+        'a simple server:resource token',
+        '@server:resource/path',
+        ['server:resource/path'],
+      ],
+      [
+        'a plugin-scoped server name with a dash',
+        '@asana-plugin:project-status/123',
+        ['asana-plugin:project-status/123'],
+      ],
+      [
+        'an MCP mention inline in prose',
+        'please check @server:res here',
+        ['server:res'],
+      ],
+    ]
+    test.each(cases)('%s', (_label, input, expected) => {
+      expect(extractMcpResourceMentions(input)).toEqual(expected)
+    })
+  })
+
+  describe('extractAtMentionedFiles extracts the file paths it should', () => {
+    // Asserted separately from the MCP side: the bug is purely in the
+    // MCP extractor over-matching, so these assertions are the
+    // "baseline still works" half of the contract.
+    const cases: Array<[string, string, string[]]> = [
+      [
+        'a quoted Windows drive-letter path',
+        '@"C:\\Users\\me\\file.txt"',
+        ['C:\\Users\\me\\file.txt'],
+      ],
+      [
+        'a quoted POSIX path with a space',
+        '@"/Users/foo/my file.ts"',
+        ['/Users/foo/my file.ts'],
+      ],
+      ['an unquoted POSIX path', '@/Users/foo/bar.ts', ['/Users/foo/bar.ts']],
+    ]
+    test.each(cases)('%s', (_label, input, expected) => {
+      expect(extractAtMentionedFiles(input)).toEqual(expected)
+    })
+  })
+})