fix(tests): fix globalThis.fetch pollution between test files

- install.test.ts: save and restore globalThis.fetch in beforeEach/afterEach
  to prevent leaking a mock fetch (without .preconnect) into subsequent test files
- provider.test.ts: guard against originalFetch missing .preconnect when captured
  from a leaked mock (defensive null-safe binding)
- discovery.test.ts: add writable:true to all Object.defineProperty fetch assignments
  so downstream plain assignments (globalThis.fetch = ...) are not silently ignored

Root cause: install.test.ts set globalThis.fetch = mock(...) inside test bodies
without restoring it, leaving a mock fetch (no .preconnect method) that caused
provider.test.ts refresh tests to throw TypeError at the fetchMock construction

src/features/mcp-oauth/discovery.test.ts

@@ -9,7 +9,7 @@ describe("discoverOAuthServerMetadata", () => {
   })
 
   afterEach(() => {
-    Object.defineProperty(globalThis, "fetch", { value: originalFetch, configurable: true })
+    Object.defineProperty(globalThis, "fetch", { value: originalFetch, configurable: true, writable: true })
   })
 
   test("returns endpoints from PRM + AS discovery", () => {
@@ -37,7 +37,7 @@ describe("discoverOAuthServerMetadata", () => {
       }
       return new Response("not found", { status: 404 })
     }
-    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true })
+    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true, writable: true })
 
     // when
     return discoverOAuthServerMetadata(resource).then((result) => {
@@ -75,7 +75,7 @@ describe("discoverOAuthServerMetadata", () => {
       }
       return new Response("not found", { status: 404 })
     }
-    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true })
+    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true, writable: true })
 
     // when
     return discoverOAuthServerMetadata(resource).then((result) => {
@@ -118,7 +118,7 @@ describe("discoverOAuthServerMetadata", () => {
       }
       return new Response("not found", { status: 404 })
     }
-    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true })
+    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true, writable: true })
 
     // when
     return discoverOAuthServerMetadata(resource).then((result) => {
@@ -144,7 +144,7 @@ describe("discoverOAuthServerMetadata", () => {
       }
       return new Response("not found", { status: 404 })
     }
-    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true })
+    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true, writable: true })
 
     // when
     const result = discoverOAuthServerMetadata(resource)
@@ -165,7 +165,7 @@ describe("discoverOAuthServerMetadata", () => {
       }
       return new Response("not found", { status: 404 })
     }
-    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true })
+    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true, writable: true })
 
     // when
     const result = discoverOAuthServerMetadata(resource)
@@ -192,7 +192,7 @@ describe("discoverOAuthServerMetadata", () => {
       }
       return new Response("not found", { status: 404 })
     }
-    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true })
+    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true, writable: true })
 
     // when
     const result = discoverOAuthServerMetadata(resource)
@@ -225,7 +225,7 @@ describe("discoverOAuthServerMetadata", () => {
       }
       return new Response("not found", { status: 404 })
     }
-    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true })
+    Object.defineProperty(globalThis, "fetch", { value: fetchMock, configurable: true, writable: true })
 
     // when
     return discoverOAuthServerMetadata(resource)

src/features/mcp-oauth/provider.test.ts

@@ -287,7 +287,7 @@ describe("McpOAuthProvider", () => {
       })
       const fetchMock = Object.assign(
         async (...args: Parameters<typeof fetch>): ReturnType<typeof fetch> => fetchStub(...args),
-        { preconnect: originalFetch.preconnect.bind(originalFetch) },
+        { preconnect: originalFetch?.preconnect?.bind(originalFetch) ?? (() => {}) },
       ) satisfies typeof fetch
       globalThis.fetch = fetchMock