fix(tui): right-click copies selection, only pastes when no selection

Sub-issue 5 of #22034.

Right-click on the composer always pasted from the clipboard, even when
the user had highlighted text — diverging from terminal-native behavior
(xterm/iTerm/gnome-terminal) where right-click copies an active selection
and only pastes when nothing is selected.

Extract a small pure helper, decideRightClickAction(value, range), and
route the existing onMouseDown right-click branch through it. Selection
present and non-empty -> writeClipboardText(slice). Otherwise fall back
to the existing emitPaste path.

ui-tui/src/__tests__/textInputRightClick.test.ts

@@ -0,0 +1,48 @@
+import { describe, expect, it } from 'vitest'
+
+import { decideRightClickAction } from '../components/textInput.js'
+
+describe('decideRightClickAction', () => {
+  it('returns paste when there is no selection', () => {
+    expect(decideRightClickAction('hello world', null)).toEqual({ action: 'paste' })
+  })
+
+  it('returns paste for a collapsed (empty) range', () => {
+    expect(decideRightClickAction('hello world', { end: 5, start: 5 })).toEqual({
+      action: 'paste'
+    })
+  })
+
+  it('copies the slice when range covers non-empty text', () => {
+    expect(decideRightClickAction('hello world', { end: 5, start: 0 })).toEqual({
+      action: 'copy',
+      text: 'hello'
+    })
+  })
+
+  it('copies a middle slice', () => {
+    expect(decideRightClickAction('hello world', { end: 11, start: 6 })).toEqual({
+      action: 'copy',
+      text: 'world'
+    })
+  })
+
+  it('falls back to paste when slice is empty (out-of-range indices)', () => {
+    expect(decideRightClickAction('', { end: 5, start: 0 })).toEqual({ action: 'paste' })
+  })
+
+  it('handles unicode (emoji, CJK) in the slice', () => {
+    const value = 'hi 你好 🎉'
+    expect(decideRightClickAction(value, { end: 5, start: 3 })).toEqual({
+      action: 'copy',
+      text: '你好'
+    })
+  })
+
+  it('preserves leading/trailing whitespace in the copied slice', () => {
+    expect(decideRightClickAction('  spaced  ', { end: 10, start: 0 })).toEqual({
+      action: 'copy',
+      text: '  spaced  '
+    })
+  })
+})