test(coverage): exercise extractTool/extractExternalTools error paths

Adds tests that mock process.smol.mount to drive:
- extractTool: mount-rejection wrapped as "failed to extract … from the SEA VFS"
  for both npm-package (cdxgen) and standalone-binary (sfw) tools
- extractExternalTools: null returns for non-SEA mode, missing mount,
  and max-depth recursion guard

dlx/vfs-extract.mts: 17.97% → 28.11% statements (14.95% → 28.97% branch).
Remaining gap (~70%) is the lock-file / extraction-recursion / TOCTOU
loop body — would require a real SEA build with process.smol or
heavy filesystem-state harnessing to exercise.

Author: jdalton

packages/cli/test/unit/utils/dlx/vfs-extract.test.mts

@@ -23,6 +23,7 @@ vi.mock('../../../../src/constants/paths.mts', () => ({
 import {
   areExternalToolsAvailable,
   EXTERNAL_TOOLS,
+  extractExternalTools,
   extractTool,
   getNodeSmolBasePath,
   getToolFilePath,
@@ -243,4 +244,36 @@ describe('utils/dlx/vfs-extract', () => {
       }
     })
   })
+
+  describe('extractExternalTools', () => {
+    it('returns null when not running in SEA mode', async () => {
+      mockIsSeaBinary.mockReturnValue(false)
+      const result = await extractExternalTools()
+      expect(result).toBeNull()
+    })
+
+    it('returns null when smol.mount is missing', async () => {
+      mockIsSeaBinary.mockReturnValue(true)
+      ;(process as any).smol = { otherProp: true }
+      try {
+        const result = await extractExternalTools()
+        expect(result).toBeNull()
+      } finally {
+        delete (process as any).smol
+      }
+    })
+
+    it('returns null when max recursion depth exceeded', async () => {
+      mockIsSeaBinary.mockReturnValue(true)
+      ;(process as any).smol = { mount: vi.fn() }
+      try {
+        // depth=5 hits the MAX_EXTRACTION_DEPTH guard and returns null
+        // before reaching any I/O.
+        const result = await extractExternalTools(5)
+        expect(result).toBeNull()
+      } finally {
+        delete (process as any).smol
+      }
+    })
+  })
 })