chore: update .fernignore, remove browser tests, and fix type definitions

- Add AGENTS.md to .fernignore to prevent Fern modifications
- Remove browser test configuration from jest.config.mjs
- Update ManagementClient type signature to include null alongside undefined

Author: developerkunal

.fernignore

@@ -20,6 +20,7 @@ src/management/tests/unit/request-options.test.ts
 tests/
 v5_MIGRATION_GUIDE.md
 CHANGELOG.md
+AGENTS.md
 .version
 docs/
 

jest.config.mjs

@@ -34,17 +34,6 @@ export default {
             testPathIgnorePatterns: ["\.browser\.(spec|test)\.[jt]sx?$", "/tests/wire/"],
             setupFilesAfterEnv: [],
         },
-        {
-            displayName: "browser",
-            preset: "ts-jest",
-            testEnvironment: "<rootDir>/src/management/tests/BrowserTestEnvironment.ts",
-            moduleNameMapper: {
-                "^(\\.{1,2}/.*)\\.js$": "$1",
-            },
-            roots: ["<rootDir>/src/management/tests"],
-            testMatch: ["<rootDir>/src/management/tests/unit/**/?(*.)+(browser).(spec|test).[jt]s?(x)"],
-            setupFilesAfterEnv: [],
-        },
         {
             displayName: "wire",
             preset: "ts-jest",

src/management/wrapper/ManagementClient.ts

@@ -253,7 +253,7 @@ function isClientOptionsWithToken(
  */
 function createTelemetryHeaders(
     _options: ManagementClientConfig,
-): Record<string, string | core.Supplier<string | undefined> | undefined> {
+): Record<string, string | core.Supplier<string | null | undefined> | null | undefined> {
     const headers = { ...(_options.headers ?? {}) };
 
     if (_options.telemetry !== false) {