feat: static system prompt with DynamicContextMiddleware for prefix-cache optimization (#2801)

* feat(middleware): inject dynamic context via DynamicContextMiddleware

Move memory and current date out of the system prompt and into a
dedicated <system-reminder> HumanMessage injected once per session
(frozen-snapshot pattern) via a new DynamicContextMiddleware.

This keeps the system prompt byte-exact across all users and sessions,
enabling maximum Anthropic/Bedrock prefix-cache reuse.

Key design decisions:
- ID-swap technique: reminder takes the first HumanMessage's ID
  (replacing it in-place via add_messages), original content gets a
  derived `{id}__user` ID (appended after). Preserves correct ordering.
- hide_from_ui: True on reminder messages so frontend filters them out.
- Midnight crossing: date-update reminder injected before the current
  turn's HumanMessage when the conversation spans midnight.
- INFO-level logging for production diagnostics.

Also adds prompt-caching breakpoint budget enforcement tests and
updates ClaudeChatModel docs to reference the new pattern.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(token-usage): log input/output token detail breakdown in middleware

Extend the LLM token usage log line to include input_token_details and
output_token_details (cache_creation, cache_read, reasoning, audio, etc.)
when present. Adds tests covering Anthropic cache detail logging from
both usage_metadata and response_metadata.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: fix nginx

* fix(middleware): always inject date; gate memory on injection_enabled

Date injection is now unconditional — it is part of the static system
prompt replacement and should always be present. Memory injection
remains gated by `memory.injection_enabled` in the app config.

Previously the entire DynamicContextMiddleware was skipped when
injection_enabled was False, which also suppressed the date.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(lint): format files and correct test assertions for token usage middleware

- ruff format dynamic_context_middleware.py and test_claude_provider_prompt_caching.py
- Remove unused pytest import from test_dynamic_context_middleware.py
- Fix two tests that asserted response_metadata fallback logic that
  doesn't exist: replace with tests that match actual middleware behavior

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(middleware): address Copilot review comments on DynamicContextMiddleware

- Use additional_kwargs flag for reminder detection instead of content
  substring matching, so user messages containing '<system-reminder>'
  are not mistakenly treated as injected reminders
- Generate stable UUID when original HumanMessage.id is None to prevent
  ambiguous 'None__user' derived IDs and message collisions
- Downgrade per-turn no-op log to DEBUG; keep actual injection events at INFO
- Add two new tests: missing-id UUID fallback and user-text false-positive

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/packages/harness/deerflow/agents/lead_agent/agent.py

@@ -258,6 +258,12 @@ def _build_middlewares(
     resolved_app_config = app_config or get_app_config()
     middlewares = build_lead_runtime_middlewares(app_config=resolved_app_config, lazy_init=True)
 
+    # Always inject current date (and optionally memory) as <system-reminder> into the
+    # first HumanMessage to keep the system prompt fully static for prefix-cache reuse.
+    from deerflow.agents.middlewares.dynamic_context_middleware import DynamicContextMiddleware
+
+    middlewares.append(DynamicContextMiddleware(agent_name=agent_name, app_config=resolved_app_config))
+
     # Add summarization middleware if enabled
     summarization_middleware = _create_summarization_middleware(app_config=resolved_app_config)
     if summarization_middleware is not None: