test: keep Codecov coverage on real agent communication paths

PR #369 was merged before the final Codecov coverage fix landed, so this follow-up carries only the incremental real-path tests needed on top of main. The tests exercise AgentSummary lifecycle branches, mailbox fail-closed behavior, UDS client connection failure through a real capability file, and UDS response-reader framing without mock.module, warning suppression, feature fallback, or production-code churn.

Constraint: PR #369 is already merged; this branch must contain only the incremental Codecov repair on top of latest main

Rejected: Reopen or keep pushing the merged PR branch | merged PR refs do not update and would leave Codecov stale

Rejected: Mock bun:bundle or hide warnings | would reintroduce cross-test pollution and pseudo coverage

Rejected: Keep unrelated SendMessageTool production diff | it created avoidable patch-coverage debt without improving the runtime path

Confidence: high

Scope-risk: narrow

Directive: Keep these coverage tests on real paths; do not replace them with output suppression or feature-flag mocks

Tested: bunx tsc --noEmit --pretty false

Tested: bun run lint

Tested: bun test src\utils\__tests__\teammateMailbox.test.ts

Tested: bun test src\services\AgentSummary\__tests__\agentSummary.test.ts src\services\AgentSummary\__tests__\summaryContext.test.ts src\utils\__tests__\teammateMailbox.test.ts src\utils\__tests__\udsMessaging.test.ts src\utils\__tests__\udsResponseReader.test.ts packages\builtin-tools\src\tools\SendMessageTool\__tests__\udsRecipientSanitization.test.ts

Tested: bun run test:all

Tested: bun test --coverage --coverage-reporter lcov --coverage-dir coverage

Tested: bun run build

Tested: bun run build:vite

Tested: bun audit

Tested: git diff --check

Tested: Claude simplify review GO (.omx/artifacts/claude-simplify-codecov-20260427-1521.md)

Tested: Claude security review GO (.omx/artifacts/claude-security-codecov-20260427-1522.md)

Not-tested: GitHub-hosted Codecov upload after this amended commit until PR checks rerun

src/utils/__tests__/teammateMailbox.test.ts

@@ -171,6 +171,17 @@ describe('compactMailboxMessages', () => {
 
     expect(compacted).toEqual([])
   })
+
+  test('returns an empty mailbox when all retention lanes are disabled', () => {
+    const compacted = compactMailboxMessages([message('unread', false)], {
+      maxMessages: 0,
+      maxReadMessages: 0,
+      maxUnreadProtocolMessages: 0,
+      maxRetainedBytes: 1_000,
+    })
+
+    expect(compacted).toEqual([])
+  })
 })
 
 describe('teammate mailbox retention', () => {
@@ -331,6 +342,26 @@ describe('teammate mailbox retention', () => {
     expect(await readFile(inboxPath, 'utf-8')).toBe('{not-json')
   })
 
+  test('writeToMailbox rejects when the inbox path is already a directory', async () => {
+    const inboxPath = getInboxPath('worker', 'alpha')
+    await mkdir(inboxPath, { recursive: true })
+
+    const error = await writeToMailbox(
+      'worker',
+      {
+        from: 'team-lead',
+        text: 'new',
+        timestamp: new Date(5).toISOString(),
+      },
+      'alpha',
+    ).then(
+      () => undefined,
+      error => error as NodeJS.ErrnoException,
+    )
+
+    expect(error?.code).toBe('EISDIR')
+  })
+
   test('readMailbox fails closed on corrupt mailbox content', async () => {
     const inboxPath = getInboxPath('worker', 'alpha')
     await mkdir(dirname(inboxPath), { recursive: true })

src/utils/__tests__/udsMessaging.test.ts

@@ -217,6 +217,23 @@ describe('UDS inbox retention', () => {
     )
   })
 
+  test('udsClient send reports connection failures without leaking token state', async () => {
+    const path = socketPath('uds-client-connect-error')
+    const capabilityDir = join(tempConfigDir, 'messaging-capabilities')
+    const capabilityName = `${createHash('sha256').update(path).digest('hex')}.json`
+    await mkdir(capabilityDir, { recursive: true, mode: 0o700 })
+    await writeFile(
+      join(capabilityDir, capabilityName),
+      JSON.stringify({ socketPath: path, authToken: 'test-token' }),
+      'utf-8',
+    )
+    const { sendToUdsSocket } = await import('../udsClient.js')
+
+    await expect(sendToUdsSocket(path, 'hello')).rejects.toThrow(
+      'Failed to connect to peer',
+    )
+  })
+
   test('sendUdsMessage fails closed before connecting without an auth token', async () => {
     await expect(
       sendUdsMessage(socketPath('no-auth-token'), { type: 'text', data: 'x' }),

src/utils/__tests__/udsResponseReader.test.ts

@@ -97,6 +97,28 @@ describe('attachUdsResponseReader', () => {
     expect(socket.ended).toBe(true)
   })
 
+  test('continues scanning when blank and valid frames share one chunk', () => {
+    const socket = new FakeSocket()
+    let settled = false
+    let settledError: Error | undefined
+
+    attachUdsResponseReader(asSocket(socket), {
+      maxFrameBytes: 128,
+      onSettled: error => {
+        settled = true
+        settledError = error
+      },
+    })
+
+    socket.emitData(
+      Buffer.from(`\n${JSON.stringify({ type: 'response' })}\n`),
+    )
+
+    expect(settled).toBe(true)
+    expect(settledError).toBeUndefined()
+    expect(socket.ended).toBe(true)
+  })
+
   test('rejects receiver error frames', () => {
     const socket = new FakeSocket()
     let settledError: Error | undefined
@@ -116,6 +138,31 @@ describe('attachUdsResponseReader', () => {
     expect(socket.destroyed).toBe(true)
   })
 
+  test('ignores unrelated receiver frames until a terminal response arrives', () => {
+    const socket = new FakeSocket()
+    let settled = false
+    let settledError: Error | undefined
+
+    attachUdsResponseReader(asSocket(socket), {
+      maxFrameBytes: 128,
+      onSettled: error => {
+        settled = true
+        settledError = error
+      },
+    })
+
+    socket.emitData(
+      Buffer.from(
+        `${JSON.stringify({ type: 'notification', data: 'queued' })}\n`,
+      ),
+    )
+    expect(settled).toBe(false)
+
+    socket.emitData(Buffer.from(`${JSON.stringify({ type: 'response' })}\n`))
+    expect(settled).toBe(true)
+    expect(settledError).toBeUndefined()
+  })
+
   test('uses custom socket error formatting', () => {
     const socket = new FakeSocket()
     let settledError: Error | undefined