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.md

@@ -13,9 +13,12 @@ Installing `deepseek-harness` installs the exact same-version `deepseek-harness-
 ```py
 from deepseek_harness import DeepSeekHarness
 
-result = DeepSeekHarness().run("Say hi.")
+with DeepSeekHarness() as harness:
+    result = harness.run("Say hi.")
 ```
 
+`DeepSeekHarness` keeps its lazily started runtime subprocess for reuse across calls. Use it as a context manager, as above, or call `close()` explicitly when finished.
+
 By default, the SDK launches the bundled single-file `dsh-jsonrpc-agent` executable from the `deepseek-harness-runtime-bin` package and injects that package's default configuration (the stdio JSON-RPC server, agent core, preloaded DeepSeek adapter, JSONL session persistence, local bash) via `DSH_CORDIS_CONFIG`. To run a plugin composition of your own, keep the `@deepseek-ai/dsh-jsonrpc` entry in the config and pass the Cordis config path.
 
 ```py