feat(signal adapter): add stream buffer and coalescer utilities

新增了用于流式消息处理的缓存队列和合并处理器类，实现了消息缓存、过期清理、流量合并格式化等能力，适配signal适配器的流式响应处理场景

backend/package/yuxi/channels/adapters/signal/streaming.py

@@ -0,0 +1,97 @@
+from __future__ import annotations
+
+import logging
+import time
+
+logger = logging.getLogger(__name__)
+
+STREAM_BUFFER_MAX_ENTRIES = 100
+STREAM_BUFFER_TTL = 300
+
+
+class StreamBuffer:
+    def __init__(self, max_entries: int = STREAM_BUFFER_MAX_ENTRIES, ttl: float = STREAM_BUFFER_TTL):
+        self._buffer: dict[str, tuple[str, float]] = {}
+        self._max_entries = max_entries
+        self._ttl = ttl
+
+    def get(self, chat_id: str) -> str:
+        buffered, _ = self._buffer.get(chat_id, ("", 0))
+        return buffered
+
+    def set(self, chat_id: str, content: str) -> None:
+        self._buffer[chat_id] = (content, time.monotonic())
+
+    def pop(self, chat_id: str) -> str | None:
+        entry = self._buffer.pop(chat_id, None)
+        return entry[0] if entry else None
+
+    def idle_ms(self, chat_id: str) -> float:
+        _, last_ts = self._buffer.get(chat_id, ("", 0))
+        if not last_ts:
+            return 0
+        return (time.monotonic() - last_ts) * 1000
+
+    def prune(self) -> None:
+        now = time.monotonic()
+        expired = [k for k, (_, ts) in self._buffer.items() if now - ts > self._ttl]
+        for k in expired:
+            self._buffer.pop(k, None)
+
+        if len(self._buffer) > self._max_entries:
+            sorted_keys = sorted(
+                self._buffer.keys(),
+                key=lambda k: self._buffer[k][1],
+            )
+            excess = len(self._buffer) - self._max_entries
+            for k in sorted_keys[:excess]:
+                self._buffer.pop(k, None)
+
+    def clear(self) -> None:
+        self._buffer.clear()
+
+
+class StreamCoalescer:
+    def __init__(
+        self,
+        buffer: StreamBuffer,
+        min_chars: int = 1500,
+        idle_ms: int = 1000,
+        lane_separator: str = "---",
+    ):
+        self._buffer = buffer
+        self._min_chars = min_chars
+        self._idle_ms = idle_ms
+        self._lane_separator = lane_separator
+
+    def should_flush_on_idle(self, chat_id: str) -> bool:
+        buffered = self._buffer.get(chat_id)
+        if not buffered:
+            return False
+        return self._buffer.idle_ms(chat_id) >= self._idle_ms
+
+    def should_flush_on_size(self, chat_id: str, new_chunk: str = "") -> bool:
+        accumulated = self._buffer.get(chat_id) + new_chunk
+        return len(accumulated) >= self._min_chars
+
+    def format_reasoning(self, chunk: str) -> str:
+        reasoning_content = chunk.replace("lane:reasoning:", "")
+        sep = self._lane_separator
+        return f"(thinking)\n{reasoning_content}\n{sep}\n"
+
+    @staticmethod
+    def is_reasoning_chunk(chunk: str) -> bool:
+        return "lane:reasoning:" in chunk
+
+
+class ReasoningLaneFormatter:
+    def __init__(self, lane_separator: str = "---"):
+        self._lane_separator = lane_separator
+
+    def format(self, chunk: str) -> str:
+        reasoning_content = chunk.replace("lane:reasoning:", "")
+        return f"(thinking)\n{reasoning_content}\n{self._lane_separator}\n"
+
+    @staticmethod
+    def is_reasoning(chunk: str) -> bool:
+        return "lane:reasoning:" in chunk