[BUG] Do not silently ignore use_shared_prefill with continuous batching

[nathon-lee]

Context

HybridEngineRollout supports both shared prompt prefill and continuous
batching, but their interaction is currently undefined.

When continuous batching is enabled through
SamplingConfig.continuous_batch_size, the continuous path is selected before
the shared-prefill logic. As a result, HybridEngineRolloutConfig.use_shared_prefill
is silently ignored.

Continuous batching currently requires n_samples_per_prompt=1, so shared
prefill does not provide a clear benefit in this mode. However, silently
ignoring the configuration can be confusing and may hide an invalid combination
of options.

Proposed work

Define and document the behavior of use_shared_prefill when continuous
batching is enabled.

Possible choices include:

• explicitly reject the combination with a clear ValueError; or
• define and implement compatible shared-prefill semantics for continuous
  batching.

Acceptance criteria

• The interaction between use_shared_prefill and continuous batching is
  explicitly defined.
• The configuration is not silently ignored.
• Tests cover both compatible and incompatible configurations.
• The default non-continuous generation path remains unchanged.

Follow-up from DeepSpeed PR #8368. cc @delock

[modelpath-dev]

I will take this issue. Please assign it to me.

The problem is that use_shared_prefill is ignored when continuous_batch_size is set. This can confuse users. I will first look at the logic in HybridEngineRollout where the continuous path is selected. I will add a check to explicitly reject the combination with a ValueError. This will make the behavior clear and prevent silent failures. I will also add tests to ensure both compatible and incompatible configurations are covered.