fix(serve-grpc): clearer error when given a new-SDK service

[saivedant169]

What does this PR address?

Fixes #5607

bentoml serve-grpc accepts any importable target, including services defined with the @bentoml.service decorator (the bentoml>=1.2 programming model). gRPC is only implemented for the legacy bentoml.Service API, so pointing serve-grpc at a new-SDK service fails with:

Error: [serve] `serve-grpc` failed: <class '_bentoml_sdk.service.factory.Service'> type doesn't support gRPC serving

The message is correct but unhelpful. The user has no way to tell from it that the new programming model has no gRPC path and that the fix is to switch transports or APIs.

Repro

# service.py
import bentoml

@bentoml.service(name="grpc_smoke")
class Supervisor:
    @bentoml.api
    def greet(self, name: str = "world") -> str:
        return f"hello {name}"

$ bentoml serve-grpc service.py:Supervisor --port 9098
Error: [serve] `serve-grpc` failed: <class '_bentoml_sdk.service.factory.Service'> type doesn't support gRPC serving

After this PR

$ bentoml serve-grpc service.py:Supervisor --port 9098
Error: [serve] `serve-grpc` failed: gRPC serving is not supported for services defined with the `@bentoml.service` decorator (the bentoml>=1.2 programming model). Use `bentoml serve` to run the service over HTTP, or rewrite it using the legacy `bentoml.Service` API if gRPC is required.

Changes

• src/bentoml/serving.py: in serve_grpc_production, detect _bentoml_sdk.Service instances explicitly and raise a BentoMLException that names the programming model and points at the two viable workarounds. Any other unrecognised type still falls through to the existing generic message.
• tests/unit/test_serve_grpc_new_sdk_error.py: writes a minimal @bentoml.service definition into a tmp dir and asserts serve_grpc_production raises the new wording.

I did not implement gRPC for the new SDK in this PR. That is a feature, not a bug fix, and the serve-grpc CLI is already hidden=True which suggests it is being phased out for new-style services.

Before submitting:

• Does the Pull Request follow Conventional Commits specification naming? Yes (fix: prefix)
• Does the code follow BentoML's code style, pre-commit run -a script has passed? Yes, ran on changed files.
• Did you read through contribution guidelines and follow development guidelines?
• Did your changes require updates to the documentation? No, error message change only.
• Did you write tests to cover your changes?