Backport poller b544f95

[yuandrew]

What was changed

Back port #1224 (commit 64cb6ed) onto commit b544f95 to fix poller shutdown for sdk-python 1.26.0

Why?

Back port poller shutdown fix to affected SDKs

Checklist

1. How was this tested: CI
2. Any docs updates needed? No

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Note

Medium Risk
Touches core worker shutdown/polling concurrency and RPC timing; mistakes could cause hangs or premature poll termination, though changes are targeted and covered by new unit/integration tests.

Overview
Fixes a graceful shutdown deadlock by moving the ShutdownWorker RPC to Worker::initiate_shutdown (spawned and then awaited during Worker::shutdown) so the server can flush in-flight long polls.

Updates poller streams to treat empty poll responses after shutdown as a termination signal (instead of a timeout to retry), and refactors namespace capability flags into a shared NamespaceCapabilities struct exposed via get_namespace_capabilities().

Adds targeted unit/core tests and new shared integration tests that start active timer+activity workflows and assert shutdown completes quickly without workflow task failures; also bumps the integration-test CLI override to v1.6.3-serverless and enables frontend.enableCancelWorkerPollsOnShutdown in dev server args.

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