[FEATURE] summarizedContext cannot be used with a streaming chat model

If an agent is configured with streamingChatModel (or streamingChatModelProvider) and also uses summarizedContext, the build fails with "A ChatModel is required to summarize context for agent X". AgentBuilder only wires up a summarizer when model or chatModelProvider is set, so there's no path for streaming agents at all.

I hit this while working on #6245 and left it alone since it was out of scope there.

Not sure what the right fix is. Simplest is probably letting you set a separate model just for summarization, something like summarizerModel(...), which seems useful on its own since you'd often want a cheaper model for that anyway. The other option is adapting the streaming model to a blocking call internally, but that hides a decision users should probably make themselves.

Worth doing? Happy to put up a PR if you have a preference.