Stress tests: two-tier channel selection for uniform type coverage

[Copilot]

Description

Local server stress tests picked channels uniformly from all instances, causing skew toward common types. With 100 SharedMaps and 10 SharedStrings, SharedMap had 10x selection probability.

Changed to two-tier selection:

1. Pick channel type uniformly
2. Pick instance from selected type

Before:

const channels = await datastore.StressDataObject.getChannels();
const channel = state.random.pick(channels);

After:

const channelsByType = new Map<string, IChannel[]>();
for (const channel of channels) {
    const channelType = channel.attributes.type;
    if (!channelsByType.has(channelType)) {
        channelsByType.set(channelType, []);
    }
    channelsByType.get(channelType)!.push(channel);
}

const selectedType = state.random.pick(Array.from(channelsByType.keys()));
const channel = state.random.pick(channelsByType.get(selectedType)!);

Modified: packages/test/local-server-stress-tests/src/localServerStressHarness.ts (mixinClientSelection function)

Reviewer Guidance

Single function change in test harness. No new test infrastructure needed - existing stress test suite validates behavior. Change maintains same invariants as original (assumes channels exist).

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