Improve TUI rendering with thinking blocks and interleaved thinking

[krissetto]

Misc rendering improvements related to thinking and interleaved thinking:

• Show agent name only when the agent actually changes, not when passing between thinking/regular content/tool calls
• Keep thinking indentation consistent with content and tool calls
• Make Thinking: a proper badge with its own styling to differentiate from the thinking content
• Show thinking badge only when its the first thinking block, or when passing from normal content to a thought. Don't show it when going between thoughts and tool calls when interleaved thinking is enabled
• Remove double newlines after thinking content

---

Screenshots

Before:

---

After: