Add worker_poll_complete_on_shutdown namespace capability (#719)

## What Changed
Adds a new namespace capability `worker_poll_complete_on_shutdown` to
indicate whether the server supports server-side completion of
outstanding worker polls on shutdown.

## Why
When this capability is enabled, the server will complete polls for
workers that:
1. Send `WorkerInstanceKey` in their poll requests
2. Call `ShutdownWorker` with the same `WorkerInstanceKey`

SDKs can check this capability via `DescribeNamespace` to decide whether
to rely on server-side poll completion or handle it client-side.

## How
- Added `bool worker_poll_complete_on_shutdown = 8` to
`NamespaceInfo.Capabilities` message in `namespace/v1/message.proto`

## Related PRs
- Server: temporalio/temporal#9323

Author: rkannan82

openapi/openapiv2.json

@@ -13670,6 +13670,10 @@
         "standaloneActivities": {
           "type": "boolean",
           "title": "True if the namespace supports standalone activities"
+        },
+        "workerPollCompleteOnShutdown": {
+          "type": "boolean",
+          "description": "True if the namespace supports server-side completion of outstanding worker polls on shutdown.\nWhen enabled, the server will complete polls for workers that send WorkerInstanceKey in their\npoll requests and call ShutdownWorker with the same WorkerInstanceKey. The poll will return\nan empty response. When this flag is true, workers should allow polls to return gracefully\nrather than terminating any open polls on shutdown."
         }
       },
       "description": "Namespace capability details. Should contain what features are enabled in a namespace."

openapi/openapiv3.yaml

@@ -10744,6 +10744,14 @@ components:
         standaloneActivities:
           type: boolean
           description: True if the namespace supports standalone activities
+        workerPollCompleteOnShutdown:
+          type: boolean
+          description: |-
+            True if the namespace supports server-side completion of outstanding worker polls on shutdown.
+             When enabled, the server will complete polls for workers that send WorkerInstanceKey in their
+             poll requests and call ShutdownWorker with the same WorkerInstanceKey. The poll will return
+             an empty response. When this flag is true, workers should allow polls to return gracefully
+             rather than terminating any open polls on shutdown.
       description: Namespace capability details. Should contain what features are enabled in a namespace.
     NamespaceInfo_Limits:
       type: object

temporal/api/namespace/v1/message.proto

@@ -42,6 +42,12 @@ message NamespaceInfo {
         bool workflow_pause = 6;
         // True if the namespace supports standalone activities
         bool standalone_activities = 7;
+        // True if the namespace supports server-side completion of outstanding worker polls on shutdown.
+        // When enabled, the server will complete polls for workers that send WorkerInstanceKey in their
+        // poll requests and call ShutdownWorker with the same WorkerInstanceKey. The poll will return
+        // an empty response. When this flag is true, workers should allow polls to return gracefully
+        // rather than terminating any open polls on shutdown.
+        bool worker_poll_complete_on_shutdown = 8;
     }
 
     // Namespace configured limits