[Feature] Add unit tests for RunExponent command flow

[ConnorQi01]

Summary

The RunExponent command (src/extension/commands/runExponent.ts) has no unit test coverage. It is the only Run* command without a dedicated test file, while RunAndroid, RunIOS, RunEasBuild, and RunMacOS/RunWindows (tracked in #2882) already have or are getting tests.

Why this is useful

Unit tests for RunExponent will catch regressions in the Expo packager startup flow and the login step (onBeforeExecute) before they reach users. The command has two distinct code paths that benefit from isolation testing: the packager lifecycle (beforeStartPackager → startPackager) and the Expo login fallback on error.

Suggested scope

• Mock ProjectVersionHelper.getReactNativePackageVersionsFromNodeModules, ExponentPlatform, loginToExponent, and ReactNativeCommand base class using the same proxyquire / Sinon pattern used in runAndroid.test.ts and runIos.test.ts.
• Cover the happy path: version resolution → ExponentPlatform constructor called with correct PlatformType.Exponent → beforeStartPackager → startPackager.
• Cover onBeforeExecute: loginToExponent is called; verify that a login failure propagates as a rejection.
• Verify that setReactNativeVersions is called with the resolved versions.

Evidence

• src/extension/commands/runExponent.ts — 30-line implementation with two-phase flow (login + packager start).
• test/extension/commands/runAndroid.test.ts, runIos.test.ts — established patterns for mocking ReactNativeCommand and platform constructors.
• Issues Add RunAndroid command flow tests #2857, [Feature] Add unit tests for RunIOS command flow #2862, [Feature] Add unit tests for RunMacOS and RunWindows command flow #2882 — ongoing sprint adding Run* command tests; runExponent is the remaining gap.

Validation

• npm run test passes with the new test file.
• Test suite reports coverage for runExponent.ts baseFn and onBeforeExecute branches.