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

---

Note

Medium Risk
Changes worker shutdown sequencing and poll stream termination semantics, which can affect how in-flight polls and shutdown are coordinated across SDKs. Risk is moderate because it touches core worker lifecycle and long-poll handling, but is covered by added unit/integration tests.

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

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

Adjusts gRPC test helpers/callback-based service to include grpc-status: 0 trailers, bumps integration CLI override to v1.6.3-serverless, and adds targeted unit/integration tests covering the new shutdown/poll termination behavior (including enabling frontend.enableCancelWorkerPollsOnShutdown).

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