fix(tui): add regression test verifying viewport auto-follows streaming answer (#5456)

[ttmouse]

修复内容

为 Issue #5456 添加回归测试，验证视口在流式输出时自动跟随。

变更说明

• 新增 —
• 测试序列：填充溢出内容 → 触发流式 event.Text → 验证视口始终保持在底部
• 补充了现有 的逆命题

验证

• go test ./internal/cli/ 全部通过
• 三种代码走查全部通过（内置 review / ponytail-review / pr-review）
• 最小改动：仅新增一个测试文件（46 行）

[SivanCola]

Thanks for adding coverage here.

I do not think this PR should be treated as the fix or regression guard for #5456 in its current form. The issue's root cause was the removal of v.MouseMode = tea.MouseModeCellMotion from the alt-screen TUI path, which prevented terminals from delivering wheel input as MouseWheelMsg events to the transcript viewport. That was restored in #5378 and then hardened further in #5400, both of which are included in v1.13.0.

I also checked the proposed test against the v1.12.0 code plus this test file: it still passes there, so it would not have caught the reported #5456 regression.

If we keep this PR, I think it should be scoped as supplemental coverage for answer-stream auto-follow while already pinned to the bottom, and the title/body should remove the #5456 fix implication. Otherwise, I think it is reasonable to close this as superseded by #5378/#5400.