DynamicContextMiddleware (PR #3630, p0) uses the ID-swap technique to
inject a SystemMessage(reminder) into the middle of the conversation.
create_agent then prepends the static system_prompt as another
SystemMessage at request time, so strict OpenAI-compatible backends
(vLLM, SGLang, Qwen) and Anthropic reject the request with
'System message must be at the beginning'.
New SystemMessageCoalescingMiddleware runs in wrap_model_call — after
create_agent prepends system_prompt — and merges every SystemMessage
into a single leading one before the request reaches the provider.
Non-system messages keep their original order; the merged SystemMessage
preserves the id of the first system message. Only the request payload
is touched; checkpoint state is unchanged, so every consumer that scans
history (memory builder, journal, summarization, dynamic-context
detection) keeps working.
Mirrors the per-request coalescing already done for Claude in
claude_provider._coalesce_system_messages (PR #3702) but at a
provider-agnostic layer so every backend benefits from a single fix.
Closes#3707
