Fix server perf contract for activity poll cache

[rmcdaniel]

Summary

• add the activity poll request cache family to the server perf harness policy inventory
• update the smoke and soak workflow threshold maps to include the new bounded-growth policy ID
• trigger the Server Perf workflow when ActivityTaskPollRequestStore changes and document the added cache surface

Validation

• tmp=/home/lab/workspace-hq/.tmp/server-perf-contract && rm -rf "$tmp" && git clone --depth=1 --branch v2 https://github.com/durable-workflow/workflow.git "$tmp/workflow-package"
• tar --exclude=.git --exclude=vendor --exclude=build -cf - . | docker run --rm -i -v "$tmp/workflow-package:/workflow:ro" -w /app composer:2 sh -lc 'tar -xf - -C /app && composer install --no-interaction --no-progress --prefer-dist && vendor/bin/phpunit tests/Unit/BoundedGrowthPolicyTest.php tests/Unit/ServerPerfHarnessContractTest.php --colors=never'
• result: OK (30 tests, 1646 assertions)