fix: handle empty Bedrock stream response (messageStop without messageStart)

[pfolegmalkov]

Problem

When using AWS Bedrock Converse API, the model may return a modelMessageStopEvent without a preceding modelMessageStartEvent. This happens when the model produces an empty response — for example, after processing certain tool results (like ask_clarifying_questions) where the model determines it has nothing to add.

The current code throws ModelError('Stream ended without completing a message') because stoppedMessage is null (set only when messageRole is truthy, which requires modelMessageStartEvent).

This is valid Bedrock behavior — the Converse API spec allows messageStop events with a stop reason even when no content blocks were produced.

Fix

In src/models/model.ts, the modelMessageStopEvent handler now defaults to 'assistant' role when modelMessageStartEvent was not received:

// Before
if (messageRole) {
  stoppedMessage = new Message({ role: messageRole, content: [...contentBlocks] })
  finalStopReason = event.stopReason!
}

// After
stoppedMessage = new Message({ role: messageRole ?? 'assistant', content: [...contentBlocks] })
finalStopReason = event.stopReason!

The contentBlocks array will be empty in this case, which is the correct representation of an empty model response.

Test plan

Added 4 new test cases in src/models/__tests__/model.test.ts:

• messageStop without messageStart → creates assistant message with empty content
• content blocks before messageStop without messageStart → preserves accumulated blocks
• toolUse stopReason without messageStart → handled correctly
• completely empty stream → still throws ModelError (no events at all = real error)

All 1617 tests pass. Coverage on model.ts branch coverage improved (87.09% → 90.32%).

Reproduction

This issue was discovered in a production conversational search application using Bedrock Converse API with Claude Sonnet 4.6 and 13 tools. The model consistently returns empty responses after ask_clarifying_questions tool results, causing the agent loop to crash.

Checklist

• Tests added
• All existing tests pass (1617/1617)
• Pre-commit hooks pass (lint, format, type-check)
• No breaking changes — empty responses that previously threw now return a valid empty Message