fix(#3189): prevent write_file streaming timeout on long reports (#3195)

* fix(#3189): prevent write_file streaming timeout on long reports

Adds a layered defense against StreamChunkTimeoutError caused by oversized
single-shot write_file tool calls:

- factory: default stream_chunk_timeout to 240s for OpenAI-compatible
  clients (overridable via ModelConfig.stream_chunk_timeout in config.yaml)
- sandbox/tools: server-side 80 KB length guard on non-append write_file
  calls (configurable via DEERFLOW_WRITE_FILE_MAX_BYTES env var, 0 disables);
  rejects oversized payloads with a structured error pointing the model at
  str_replace or append=True
- middleware: classify StreamChunkTimeoutError as transient but cap retries
  at 1 via per-exception _RETRY_BUDGET_OVERRIDES (same-payload retry on a
  chunk-gap timeout buffers the same way upstream; full 3-attempt loop
  would stack 6-12 min of dead air)
- middleware: surface an actionable user-facing message for stream-drop
  exceptions instead of leaking the raw langchain stack
- prompts: add a routing-style File Editing Workflow hint to both lead_agent
  and general_purpose subagent prompts, pointing the model at str_replace
  for incremental edits (mirrors Claude Code's Edit / Codex's apply_patch)
- tests: behavioural coverage for size guard, retry budget override,
  stream-drop user message, factory default injection

Refs #3189

* fix(#3189): drop stream_chunk_timeout for non-OpenAI providers

Address CR feedback on PR #3195:

- factory: pop `stream_chunk_timeout` from kwargs for any model_use_path other than `langchain_openai:ChatOpenAI` instead of returning early. `ModelConfig.stream_chunk_timeout` is part of the shared schema, so a user-supplied value on a non-OpenAI provider would otherwise be forwarded to its constructor and raise `TypeError: unexpected keyword argument`.

- factory: rewrite docstring to describe the actual `exclude_none=True` behaviour (explicit null is excluded and falls back to the default) instead of the misleading "None falling out via exclude_none=True keeps its value".

- tests: add regression coverage asserting the kwarg is stripped before reaching a non-OpenAI provider's constructor.

Refs: bytedance#3189

* fix(#3189): restrict stream-drop user copy to StreamChunkTimeoutError only

Per CR on #3195: narrow _STREAM_DROP_EXCEPTIONS to StreamChunkTimeoutError. Generic httpx RemoteProtocolError / ReadError fall back to the standard 'temporarily unavailable' copy, since they routinely fire on transient network blips where the 'split the output' guidance is misleading. Retry/backoff classification is unchanged — both remain transient/retriable. Tests updated to reflect new copy, plus a symmetric regression test for ReadError.

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/packages/harness/deerflow/subagents/builtins/general_purpose.py

@@ -24,6 +24,17 @@ Do NOT use for simple, single-step operations.""",
 - Do NOT ask for clarification - work with the information provided
 </guidelines>
 
+<file_editing_workflow>
+When revising an existing file, prefer `str_replace` over `write_file` —
+it sends only the diff and avoids re-emitting the whole file (mirrors
+Claude Code's Edit and Codex's apply_patch). When writing long new
+content from scratch, split it into sections: the first `write_file`
+call creates the file, then use `write_file` with append=True to extend
+it section by section. This keeps each tool call small and avoids
+mid-stream chunk-gap timeouts on oversized single-shot writes.
+(See issue #3189.)
+</file_editing_workflow>
+
 <output_format>
 When you complete the task, provide:
 1. A brief summary of what was accomplished