fix(agent-chat): surface a rejected send on the transcript branch

[teamleaderleo]

Follow-up to #16121. That PR made send reject where it previously did not: the Pi adapter now throws pi process changed during startup rather than writing a prompt to a process that never received its setup. One of the three sendPrompt call sites does not handle a rejection.

agent-chat/server.ts on the transcript branch was:

if (sess.transcript) {
  // Typed into the terminal's agent; the transcript records the prompt.
  void sess.adapter.send(sess, prompt);
  return;
}

A rejection there became an unhandled rejection with no feedback: the user's message vanished and the UI kept waiting. The other two call sites already catch, log and emit an error event.

The fix catches it and emits the same sanitized error event those two use. There is no turn generation on this path (the transcript records the prompt, so nothing was allocated), so there is nothing to unwind and no done to emit.

The test drives sendPromptForTest with a transcript session whose adapter rejects, and asserts both halves of the contract: an error event is emitted, and no unhandledRejection fires. It landed in its own commit first and fails on main:

error: a rejected transcript send must emit an error event: []

After the fix, transcript send rejection assertions passed.

One thing this deliberately does not change. safeReason is an allow-list, so pi process changed during startup maps to unexpected error and the user reads Failed to send message: unexpected error. The test asserts that exact string, because it is the shape the other two call sites already produce. Making the startup-race reason legible would mean widening safeReason, which is a policy call about what adapter text is allowed into the UI, and it belongs in its own change.

Validation, from agent-chat:

• ./node_modules/.bin/tsc --noEmit, clean
• bun run-tests.ts, 115 pass / 0 fail / 269 expect() calls, 39 scripts and 18 bun:test files, exit 0

Correction to something I wrote on #16121: I said the server half of server.ts had no test harness. It does. sendPromptForTest has been exported since before this work and test/server-utils.test.ts already uses it, which is what this test is built on.

🤖 Generated with Claude Code

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Summary by cubic

Follows up on #16121, where the Pi adapter started rejecting send when its process changes during startup (pi process changed during startup). The transcript branch of sendPrompt dispatched with a bare void sess.adapter.send(...), so a rejection became an unhandled rejection and the user got no feedback — the message vanished and the UI kept waiting. The other two call sites already catch, log, and emit an error event.

The fix catches the rejection and emits the same sanitized error event those two call sites use. There is no turn generation on this path (the transcript records the prompt), so there is nothing to unwind and no done to emit.

Bug Fixes

• A rejected transcript send now emits an error event and leaves no unhandledRejection.
• Adds a test driving sendPromptForTest with a rejecting adapter that asserts both halves of that contract.
• The message stays Failed to send message: unexpected error because safeReason is an allow-list; widening it is deliberately left out of this change.

^{Written for commit 47cfced. Summary will update on new commits.}

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• agent-chat/server.ts
• agent-chat/test/server-transcript-send.test.ts

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Merge receipt for 47cfced855: every check was green at merge (10 verified; 14 skipped by policy). Full suite runs on main after merge.