Test tunnel provider plugin (#4839)

### WHY are these changes introduced?

Adds test coverage for the tunnel plugin functionality to ensure reliable behavior of tunnel-related operations. This module was low on coverage.

### Measuring impact

- [x] n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix

### Checklist

- [x] I've considered possible cross-platform impacts (Mac, Linux, Windows)
- [x] I've considered possible [documentation](https://shopify.dev) changes

packages/cli-kit/src/public/node/plugins/tunnel.test.ts

@@ -0,0 +1,126 @@
+import {defineProvider, startTunnel, TunnelError} from './tunnel.js'
+import {ok} from '../result.js'
+import {describe, test, expect} from 'vitest'
+
+describe('tunnel', () => {
+  describe('defineProvider', () => {
+    test('returns a function that resolves with the provider name', async () => {
+      // Given
+      const providerName = 'test-provider'
+      const provider = defineProvider({name: providerName})
+
+      // When
+      const result = await provider.call({} as any, {} as any)
+
+      // Then
+      expect(result).toEqual({name: providerName})
+    })
+  })
+
+  describe('startTunnel', () => {
+    test('returns error when provider does not match', async () => {
+      // Given
+      const tunnelFn = startTunnel({
+        provider: 'cloudflare',
+        action: async () =>
+          ok({
+            provider: 'cloudflare',
+            port: 8080,
+            getTunnelStatus: () => ({status: 'not-started'}),
+            stopTunnel: () => {},
+          }),
+      })
+
+      // When
+      const result = await tunnelFn.call({} as any, {provider: 'wrong-provider', port: 8080, config: {} as any})
+
+      // Then
+      expect(result.isErr()).toBe(true)
+      expect((result as any).error).toBeInstanceOf(TunnelError)
+      expect((result as any).error.type).toBe('invalid-provider')
+    })
+
+    test('calls action with port when provider matches', async () => {
+      // Given
+      const port = 8080
+      const provider = 'cloudflare'
+      let calledWithPort: number | undefined
+
+      const tunnelFn = startTunnel({
+        provider,
+        action: async (receivedPort) => {
+          calledWithPort = receivedPort
+          return ok({
+            provider,
+            port: receivedPort,
+            getTunnelStatus: () => ({status: 'not-started'}),
+            stopTunnel: () => {},
+          })
+        },
+      })
+
+      // When
+      const result = await tunnelFn.call({} as any, {provider, port, config: {} as any})
+
+      // Then
+      expect(result.isErr()).toBe(false)
+      expect(calledWithPort).toBe(port)
+    })
+
+    test('returns successful tunnel client when everything succeeds', async () => {
+      // Given
+      const port = 8080
+      const provider = 'cloudflare'
+      const tunnelClient = {
+        provider,
+        port,
+        getTunnelStatus: () => ({status: 'not-started' as const}),
+        stopTunnel: () => {},
+      }
+
+      const tunnelFn = startTunnel({
+        provider,
+        action: async () => ok(tunnelClient),
+      })
+
+      // When
+      const result = await tunnelFn.call({} as any, {provider, port, config: {} as any})
+
+      // Then
+      expect(result.isErr()).toBe(false)
+      if (!result.isErr()) {
+        expect(result.value).toEqual(tunnelClient)
+        expect(result.value.getTunnelStatus()).toEqual({status: 'not-started'})
+      }
+    })
+  })
+
+  describe('TunnelError', () => {
+    test('creates error with correct type and message', () => {
+      // Given
+      const errorType = 'invalid-provider'
+      const errorMessage = 'Invalid tunnel provider'
+
+      // When
+      const error = new TunnelError(errorType, errorMessage)
+
+      // Then
+      expect(error).toBeInstanceOf(TunnelError)
+      expect(error.type).toBe(errorType)
+      expect(error.message).toBe(errorMessage)
+    })
+
+    test('creates error with only type when message is omitted', () => {
+      // Given
+      const errorType = 'unknown'
+
+      // When
+      const error = new TunnelError(errorType)
+
+      // Then
+      expect(error).toBeInstanceOf(TunnelError)
+      expect(error.type).toBe(errorType)
+      expect(error.message).toBe('')
+    })
+  })
+})