Channel hangs with 400 'text content blocks must be non-empty' when skip is called in same turn as branch tool

Description

When the Channel LLM calls the branch tool and the skip tool in the same turn (while waiting for the branch result), the resulting assistant message in conversation history contains an empty text content block. Every subsequent Anthropic API call using that history then fails permanently with:

Anthropic API error (400 Bad Request): messages: text content blocks must be non-empty

The channel becomes completely unresponsive — all new user messages fail instantly with the same error.

Reproduction Steps

1. Send a message that causes the Channel to spawn a branch and call skip in the same turn (e.g. ask for a system status check)
2. The branch completes and a retrigger fires
3. The retrigger API call fails with 400
4. All subsequent messages to the same channel fail with 400

From the logs, the exact sequence is:

branch spawned → branch tool call recorded in assistant turn
skip tool called → skip recorded in same assistant turn (empty text block created)
branch completes → retrigger fires
→ ERROR: Anthropic API error (400 Bad Request): messages: text content blocks must be non-empty

Root Cause

The Anthropic API requires that if a text content block is present in an assistant message, it must be non-empty. When skip is used alongside a branch tool call in the same turn, the message serialization produces an assistant message with an empty text block followed by tool_use blocks. This violates the API constraint.

The likely fix is to strip empty text content blocks from assistant messages before sending to the Anthropic API, or ensure the serialization never produces them.

Impact

• Channel is permanently stuck until conversation history is manually cleared and the daemon restarted
• Recovery requires: DELETE FROM conversation_messages WHERE channel_id = '<id>'; + spacebot restart

Environment

• Spacebot v0.2.0
• Channel model: anthropic/claude-haiku-4-5
• Platform: Telegram

Start by reproducing the branch-and-skip sequence and tracing assistant-message serialization through the Anthropic API call. No source file or test is named in the issue, so locate where the assistant history is serialized and verify that the retrigger and later messages no longer contain empty text content blocks.