[Test Improver] Modernize MainCoroutineRule: replace deprecated TestCoroutineDispatcher/runBlockingTest

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MainCoroutineRule.kt uses APIs deprecated since kotlinx-coroutines-test 1.6:

• TestCoroutineDispatcher → replaced by StandardTestDispatcher / UnconfinedTestDispatcher
• runBlockingTest → replaced by runTest
• cleanupTestCoroutines() → no longer needed (handled automatically)

Current code (app/unit-tests/src/org/commcare/rules/MainCoroutineRule.kt):

`@ExperimentalCoroutinesApi`
class MainCoroutineRule(val testDispatcher: TestCoroutineDispatcher = TestCoroutineDispatcher()) : TestWatcher() {
    override fun starting(description: Description) {
        super.starting(description)
        Dispatchers.setMain(testDispatcher)
    }
    override fun finished(description: Description) {
        super.finished(description)
        Dispatchers.resetMain()
        testDispatcher.cleanupTestCoroutines()   // deprecated, no-op in new API
    }
    fun runBlockingTest(block: suspend () -> Unit) =
        this.testDispatcher.runBlockingTest { block() }  // deprecated
}

Scope: 2 files total — MainCoroutineRule.kt and LazyMediaDownloadTest.kt (the only consumer).

Proposed fix:

`@OptIn`(ExperimentalCoroutinesApi::class)
class MainCoroutineRule(val testDispatcher: TestDispatcher = StandardTestDispatcher()) : TestWatcher() {
    override fun starting(description: Description) {
        super.starting(description)
        Dispatchers.setMain(testDispatcher)
    }
    override fun finished(description: Description) {
        super.finished(description)
        Dispatchers.resetMain()
    }
}

And in LazyMediaDownloadTest.kt, replace mainCoroutineRule.runBlockingTest { ... } with runTest { ... } (from kotlinx.coroutines.test).

Why now: The deprecated APIs produce compiler warnings and will be removed in a future coroutines release. Fixing this keeps the test infrastructure clean and unblocks future coroutine test authors from inheriting the deprecated pattern.

This is a small, self-contained change with no functional impact on the tested behavior.

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