fix(session): filter empty text content before sending to API

[walker83]

Summary

Filter out empty text content when converting messages to API format. This prevents the "all messages must have non-empty content" error (HTTP 400) that can occur after pruning clears reasoning content.

Problem

As reported in #1326, long-running sessions can hit a 400 error:

Bad Request: {"error":{"message":"messages.271: all messages must have non-empty content","type":"invalid_request_error"}}

The root cause is in toModelMessagesEffect:

• prune.ts:225 clears reasoning content by setting text: ""
• message-v2.ts:683,745 converts these empty-text parts to API messages without filtering
• The API rejects messages with empty content

Fix

Add part.text.length > 0 checks when building user and assistant message parts:

// Before (user messages)
if (part.type === "text" && !part.ignored)

// After
if (part.type === "text" && !part.ignored && part.text.length > 0)

// Before (assistant messages)  
if (part.type === "text")

// After
if (part.type === "text" && part.text.length > 0)

Changes

• packages/opencode/src/session/message-v2.ts: Add empty text content filter

Testing

1. Start a session and have a long conversation (>100 messages)
2. Trigger pruning (context overflow)
3. Continue the conversation
4. Verify no 400 errors occur

Related Issues

• Fixes 模型会在一些轮次后陷入400错误并彻底卡死 #1326

Checklist

• Code follows project style guidelines
• Single file change
• Backward compatible