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.

packages/ui/jsonrpc/tests/server.spec.ts

@@ -152,6 +152,57 @@ describe('HarnessSdkServer', () => {
     }
   })
 
+  it('rejects overlapping prompts for one session without serializing other sessions', async () => {
+    let releaseMain: (() => void) | undefined
+    const firstMainIdle = new Promise<void>((resolve) => { releaseMain = resolve })
+    const mainWhenIdle = vi.fn<() => Promise<void>>()
+      .mockReturnValueOnce(firstMainIdle)
+      .mockResolvedValue(undefined)
+    const mainSend = vi.fn()
+    const mainAgent = {
+      send: mainSend,
+      whenIdle: mainWhenIdle,
+    } as unknown as Agent
+    const otherSend = vi.fn()
+    const otherAgent = {
+      send: otherSend,
+      whenIdle: vi.fn(() => Promise.resolve()),
+    } as unknown as Agent
+    const mainHandle = { agent: mainAgent, dispose: vi.fn(() => Promise.resolve()) }
+    const otherHandle = { agent: otherAgent, dispose: vi.fn(() => Promise.resolve()) }
+    const create = vi.fn(async (options: { agentId: AgentId }) =>
+      String(options.agentId) === 'main' ? mainHandle : otherHandle)
+    const ctx = {
+      on: vi.fn(() => () => undefined),
+      agents: { create, get: () => undefined },
+      get: () => undefined,
+    } as unknown as Context
+    const server = new HarnessSdkServer(ctx, new FakeTransport())
+    const prompt = (sessionId: string, text: string) => server.prompt({
+      sessionId,
+      contentBlocks: [{ type: 'text', text }],
+    })
+
+    const first = prompt('main', 'first')
+    await vi.waitFor(() => { expect(mainSend).toHaveBeenCalledOnce() })
+
+    await expect(prompt('main', 'overlap')).rejects.toThrow('session already has an active prompt: main')
+    await expect(prompt('other', 'independent')).resolves.toEqual({ accepted: true })
+    releaseMain?.()
+    await expect(first).resolves.toEqual({ accepted: true })
+    await expect(prompt('main', 'sequential')).resolves.toEqual({ accepted: true })
+
+    mainWhenIdle.mockRejectedValueOnce(new Error('turn wait failed'))
+    await expect(prompt('main', 'failing')).rejects.toThrow('turn wait failed')
+    await expect(prompt('main', 'after failure')).resolves.toEqual({ accepted: true })
+
+    expect(mainSend).toHaveBeenCalledTimes(4)
+    expect(otherSend).toHaveBeenCalledOnce()
+    await server.shutdown()
+    expect(mainHandle.dispose).toHaveBeenCalledOnce()
+    expect(otherHandle.dispose).toHaveBeenCalledOnce()
+  })
+
   it('notifies the host when a child session is created with parent lineage', async () => {
     const storageDir = await mkdtemp(join(tmpdir(), 'dsh-jsonrpc-subagent-'))
     const ctx = await makeHarness(storageDir)
@@ -490,11 +541,11 @@ describe('HarnessSdkServer', () => {
       get: () => undefined,
     } as unknown as Context
     const server = new HarnessSdkServer(ctx, new FakeTransport()) as unknown as {
-      sessions: Map<string, { handle: AgentHandle; lastTurnEnd: undefined }>
+      sessions: Map<string, { handle: AgentHandle; lastTurnEnd: undefined; activePrompt: boolean }>
       shutdown(): Promise<Record<string, never>>
     }
-    server.sessions.set('first', { handle: { agent: {} as Agent, dispose: firstDispose }, lastTurnEnd: undefined })
-    server.sessions.set('second', { handle: { agent: {} as Agent, dispose: secondDispose }, lastTurnEnd: undefined })
+    server.sessions.set('first', { handle: { agent: {} as Agent, dispose: firstDispose }, lastTurnEnd: undefined, activePrompt: false })
+    server.sessions.set('second', { handle: { agent: {} as Agent, dispose: secondDispose }, lastTurnEnd: undefined, activePrompt: false })
 
     await expect(server.shutdown()).rejects.toThrow('SDK server teardown failed')
     expect(firstDispose).toHaveBeenCalledOnce()