fix(chat): preserve assistant text blocks across tool iterations

[ABB65]

Summary

Conversation loop streams assistant text deltas to SSE, but per-iteration assistantBlocks was rebuilt only from currentToolCalls when pushing into config.messages for the next Anthropic call. Two concrete regressions fell out of this:

1. Multi-turn protocol amputation. Next iteration saw prior assistant turn as [tool_use] only — Claude's own narration ("I'll check the schema first, then update X") was missing from its view. Anthropic accepts the payload but downstream tool sequencing degrades.

2. Empty done.lastContent on text-only end_turn. The streaming branch never set lastAssistantContent, so the most common chat shape (plain text reply, no tool use) reached saveChatResult with [] — DB stored the '[tool calls]' placeholder fallback (db.ts:250) instead of the actual assistant text. This silently broke chat history persistence.

Fix

Purely in-memory. assistantBlocks now allocated at the top of each iteration and populated as content blocks come in:

• Streaming branch: single text buffer accumulates text deltas, flushed on tool_use_start and message_end. tool_use_end pushes block to both currentToolCalls (for tool execution) and assistantBlocks (for protocol/persistence).
• Non-streaming branch: each block from response.content pushed directly.
• Old reconstruction at the tool-execution boundary deleted.
• lastAssistantContent = assistantBlocks assigned once outside the split, before the stopReason check — so end_turn and tool_use both yield the correct final shape.

Test plan

Four regression tests in conversation-engine-regression.test.ts:

• returns streamed text-only responses in done.lastContent — guards the empty lastContent regression that was silently producing [tool calls] placeholders
• preserves streamed assistant text before tool use in the next model message — main protocol-amputation fix
• preserves streamed interleaved text and tool_use block order — text→tool→text→tool ordering
• preserves non-streaming assistant text before tool use in later iterations — iter 2+ branch correctness
• pnpm test:unit — 408 passed (was 404)
• pnpm typecheck clean
• pnpm lint — 0 errors on changed files

Out of scope

DB persistence of intermediate tool iterations and tool_result blocks — belongs to the upcoming tool-trace-persistence PR (will introduce messages.content_blocks jsonb + iteration index).