[Bug]: Errors in server logs while deploying a service with gRPC workers

### Steps to reproduce

Create a service (the same configuration as in #4240 issue):

```yaml
type: service
name: qwen38-27b-agentic-smg

groups:
  - replicas: 1
    python: "3.12"
    commands:
      - pip install smg
      - smg launch --host 0.0.0.0 --port 8000 --enable-igw --policy cache_aware --model-path Qwen/Qwen3.8-27B
    router:
      type: sglang
    resources:
      gpu: 0

  - replicas: 1
    image: vllm/vllm-openai:qwen38
    commands:
      - pip install smg-grpc-servicer
      - python3 -m vllm.entrypoints.grpc_server --host 0.0.0.0 --port 8000 --model Qwen/Qwen3.8-27B-FP8
    resources:
      gpu: H100:80GB

port: 8000
model:
  type: chat
  name: Qwen/Qwen3.8-27B
  format: openai
  prefix: /v1
```

### Actual behaviour

There are two errors in the server logs:

1. `Parent instance <RunModel at 0x78af662b9e40> is not bound to a Session; deferred load operation of attribute 'run_name' cannot proceed` – occurs once
2. `Could not fetch server_info for worker http://10.0.0.3:8000: ReadError('')` – repeats until the worker is ready

### Expected behaviour

_No response_

### dstack version

0.22.1

### Server logs

```shell
DEBUG    dstack._internal.server.background.pipeline_tasks.base:357 Processing service_router_worker_sync item
         648bb027-2a5c-447e-80c7-a8e0b391bc37
ERROR    dstack._internal.server.background.pipeline_tasks.base:361 Unexpected exception when processing item
         Traceback (most recent call last):
           File "/home/def/dev/dstack/src/dstack/_internal/server/background/pipeline_tasks/base.py", line 359, in start
             await self.process(item)
           File "/home/def/dev/dstack/src/dstack/_internal/server/utils/tracing.py", line 26, in wrapper
             result = await f(*args, **kwargs)
           File "/home/def/dev/dstack/src/dstack/_internal/server/background/pipeline_tasks/service_router_worker_sync.py", line
         269, in process
             await sync_router_workers_for_run_model(run_for_sync)
           File "/home/def/dev/dstack/src/dstack/_internal/server/services/runs/router_worker_sync.py", line 643, in
         sync_router_workers_for_run_model
             fmt(run_model),
           File "/home/def/dev/dstack/src/dstack/_internal/server/services/logging.py", line 26, in fmt
             return fmt_entity("run", model.id, model.run_name)
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/sqlalchemy/orm/attributes.py", line 569, in __get__
             return self.impl.get(state, dict_)  # type: ignore
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/sqlalchemy/orm/attributes.py", line 1096, in get
             value = self._fire_loader_callables(state, key, passive)
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/sqlalchemy/orm/attributes.py", line 1129, in
         _fire_loader_callables
             return callable_(state, passive)
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/sqlalchemy/orm/strategies.py", line 585, in __call__
             return strategy._load_for_state(state, passive)
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/sqlalchemy/orm/strategies.py", line 543, in
         _load_for_state
             raise orm_exc.DetachedInstanceError(
         sqlalchemy.orm.exc.DetachedInstanceError: Parent instance <RunModel at 0x78af662b9e40> is not bound to a Session; deferred
         load operation of attribute 'run_name' cannot proceed (Background on this error at: https://sqlalche.me/e/20/bhk3)
DEBUG    dstack._internal.server.background.pipeline_tasks.base:364 Processed service_router_worker_sync item
         648bb027-2a5c-447e-80c7-a8e0b391bc37 in 0.022

----------------------------------

ERROR    dstack._internal.server.services.runs.router_worker_sync:421 Could not fetch server_info for worker http://10.0.0.3:8000:
         ReadError('')
         Traceback (most recent call last):
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpx/_transports/default.py", line 101, in
         map_httpcore_exceptions
             yield
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpx/_transports/default.py", line 394, in
         handle_async_request
             resp = await self._pool.handle_async_request(req)
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpcore/_async/connection_pool.py", line 256, in
         handle_async_request
             raise exc from None
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpcore/_async/connection_pool.py", line 236, in
         handle_async_request
             response = await connection.handle_async_request(
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpcore/_async/connection.py", line 103, in
         handle_async_request
             return await self._connection.handle_async_request(request)
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpcore/_async/http11.py", line 136, in
         handle_async_request
             raise exc
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpcore/_async/http11.py", line 106, in
         handle_async_request
             ) = await self._receive_response_headers(**kwargs)
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpcore/_async/http11.py", line 177, in
         _receive_response_headers
             event = await self._receive_event(timeout=timeout)
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpcore/_async/http11.py", line 217, in _receive_event
             data = await self._network_stream.read(
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpcore/_backends/anyio.py", line 32, in read
             with map_exceptions(exc_map):
           File "/home/def/.local/share/uv/python/cpython-3.10-linux-x86_64-gnu/lib/python3.10/contextlib.py", line 153, in
         __exit__
             self.gen.throw(typ, value, traceback)
           File "/home/def/dev/dstack/.venv/lib/python3.10/site-packages/httpcore/_exceptions.py", line 14, in map_exceptions
             raise to_exc(exc) from exc
         httpcore.ReadError
```

### Additional information

_No response_

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[Bug]: Errors in server logs while deploying a service with gRPC workers #4300

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Uh oh!

[Bug]: Errors in server logs while deploying a service with gRPC workers #4300

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