fix(opencode): preserve signed thinking during anthropic reorder

[rekram1-node]

Summary

• keep the existing Anthropic reorder for trailing ordinary text
• skip that reorder when it would move signed or redacted Anthropic reasoning after a tool call
• add focused coverage for signed-thinking preservation without changing existing regression expectations

Context

The existing workaround correctly handles tool_use -> text histories by putting ordinary text before tool calls. It is unsafe for tool_use -> thinking, because Anthropic thinking blocks are signed and cannot be relocated during replay.

This change narrows the workaround rather than restructuring model messages at a higher level.

Tests

• bun test test/provider/transform.test.ts
• bun test test/session/message-v2.test.ts
• bun test test/session/llm.test.ts