[Bug] TUI crashes on assistant.delta events with missing delta payload (Cannot read properties of undefined (reading 'trim'))

Environment

• kimi-code TUI on the agent-core-v2 engine (current main, also reproducible on 0.34.x)
• Model served through an Anthropic-compatible relay/proxy (not the first-party Anthropic API)

Error

The TUI throws and logs an unexpected error while streaming:

[unexpected] TypeError: Cannot read properties of undefined (reading 'trim')
    at SessionEventHandler.handleAssistantDelta (.../main.cjs)
    at SessionEventHandler.handleEvent (.../main.cjs)
    at SDKRpcClientV2.receiveEvent (.../main.cjs)

Root cause

Three small gaps line up:

1. Parser: in packages/agent-core-v2/src/kosong/provider/bases/anthropic/anthropic.ts, the stream converter trusts the SDK types: content_block_start yields { type: 'text', text: block.text } and text_delta yields { type: 'text', text: delta.text }. Some relays omit the text field entirely, so the engine produces a text part with text: undefined.
2. Transport: loopService.ts publishes { type: 'assistant.delta', turnId, delta: part.text }. When the event crosses JSON serialization, the undefined value silently drops the delta key, so the TUI receives an event with no delta at all.
3. TUI: handleAssistantDelta in apps/kimi-code/src/tui/controllers/session-event-handler.ts immediately calls event.delta.trim(), which throws on undefined.

Minimal reproduction

Have an Anthropic-compatible endpoint stream:

{"type":"content_block_start","index":0,"content_block":{"type":"text"}}
{"type":"content_block_delta","index":0,"delta":{"type":"text_delta"}}

(both omit text). Every such event crashes the TUI handler.

Suggested fix

• Parser: block.text ?? '' / delta.text ?? '' (same pattern already used for thinking ?? '' two cases below).
• TUI: coerce a non-string event.delta to '' before use, so a malformed wire record can never crash the handler again.

Happy to submit a PR with tests if the approach looks good.

Start with the Anthropic stream converter in packages/agent-core-v2/src/kosong/provider/bases/anthropic/anthropic.ts and the event handler in apps/kimi-code/src/tui/controllers/session-event-handler.ts, using the two-event reproduction as the first check. Trace the related publishing path in loopService.ts; done means missing text fields produce safe empty values and the TUI no longer crashes on malformed assistant.delta events, with tests added for the behavior.