Backport poller 71a5caa

[yuandrew]

What was changed

Back port #1224 (commit 64cb6ed) onto commit 71a5caa 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
Changes core worker shutdown sequencing and poll-stream termination behavior; mistakes could cause hangs or premature poll termination during shutdown. Covered by new unit/integration tests but touches concurrency-critical code paths.

Overview
Fixes a graceful worker shutdown deadlock by moving the ShutdownWorker RPC to Worker::initiate_shutdown (spawned in the background and awaited during shutdown()), ensuring the server is signaled to flush in-flight long polls.

Updates poll-stream logic to treat an empty poll response after shutdown has begun as a graceful termination signal (instead of a timeout that causes retry loops), and centralizes namespace capability flags into a new NamespaceCapabilities struct that is exposed via get_namespace_capabilities().

Adds targeted unit/core/integration tests and adjusts dev-server dynamic config and CLI version override to exercise and validate the new shutdown behavior.

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