fix(sdk): harden runtime lifecycle and JSON-RPC

Keep DeepSeekHarness.run() reusable, but make ownership of its lazy
runtime process explicit. Document the context-manager/close contract and
update every construction example to use a context manager so repeated runs
remain valid without encouraging leaked subprocesses.

Contain notification predicate failures at the subscription boundary. Remove
only the subscriber whose callback raised, deliver that exception through its
queue, and continue dispatching to healthy subscribers so arbitrary callback
code cannot terminate the shared reader thread or strand later requests.

Enforce one in-flight prompt per server session with an atomic activePrompt
guard. Route overlap through the existing -32603 handler-error response and
clear the guard in finally, preserving parallel prompts across sessions and
sequential reuse without changing JSON-RPC request or notification shapes.

Use StringDecoder for line framing so a UTF-8 code point split across Buffer
chunks is not corrupted. Add a queued-write flush barrier, and make memoized
shutdown await it before disposal and exit while retaining exactly-once
cleanup when shutdown calls race or flushing fails.

Cover callback isolation, same-session exclusion, cross-session concurrency,
split multibyte input, delayed writes, racing shutdown, and flush failure with
deterministic tests.

python/sdk/README.zh.md

@@ -9,9 +9,12 @@
 ```py
 from deepseek_harness import DeepSeekHarness
 
-result = DeepSeekHarness().run("Say hi.")
+with DeepSeekHarness() as harness:
+    result = harness.run("Say hi.")
 ```
 
+`DeepSeekHarness` 会保留延迟启动的运行时子进程，以供多次调用复用。请像上例一样将其用作上下文管理器，或在用完后显式调用 `close()`。
+
 默认情况下，SDK 启动 `deepseek-harness-runtime-bin` 包内置的单文件 `dsh-jsonrpc-agent` 可执行程序，并通过 `DSH_CORDIS_CONFIG` 注入该包的默认配置（stdio JSON-RPC 服务器、agent core、预载的 DeepSeek 适配器、JSONL 会话持久化、本地 bash）。要运行自己用插件组合需要在配置里保留 `@deepseek-ai/dsh-jsonrpc` 条目，并传入 Cordis 配置路径。
 
 ```py