Fix flaky integration test and clean up test patterns

[Copilot]

The "Change event includes old and new values" test had a race condition: setEnvironment(undefined, oldEnv) fires an async event that could be captured by the TestEventHandler registered immediately after, causing the handler to see the wrong event.

Changes

• Fix race condition: Add waitForCondition between the initial setEnvironment and handler registration to ensure the first event has fully propagated
• Eliminate duplicate getEnvironment calls: The original pattern called waitForCondition (polling getEnvironment until ready) then immediately re-fetched and asserted on a second getEnvironment call. Combined these by capturing the result inside the waitForCondition callback:

// Before: fetch twice
await waitForCondition(async () => {
    const e = await api.getEnvironment(undefined);
    return !!e && e.environmentPath.fsPath === envToSet.environmentPath.fsPath;
}, 15_000, ...);
const retrieved = await api.getEnvironment(undefined); // redundant
assert.ok(retrieved, ...);
assert.strictEqual(retrieved.environmentPath.fsPath, ...);

// After: capture and reuse
let retrieved: PythonEnvironment | undefined;
await waitForCondition(async () => {
    retrieved = await api.getEnvironment(undefined);
    return !!retrieved && retrieved.environmentPath.fsPath === envToSet.environmentPath.fsPath;
}, 15_000, ...);
assert.ok(retrieved, ...);
assert.strictEqual(retrieved!.environmentPath.fsPath, ...);

• Replace raw setTimeout with waitForCondition in the idempotency test — a fixed 500ms sleep is fragile on slow CI runners
• Use top-level import for PythonEnvironment instead of repeated inline import('../../api').PythonEnvironment type expressions