fix(input): strip leading ! when entering bash mode (#947)

The PromptInput onChange handler had two branches for entering bash
mode: a single-char path that just toggled the mode and a multi-char
paste path that also stripped the leading `!` from the buffer. The
single-char path returned without stripping, so typing a bare `!` into
empty input switched modes but left the literal `!` visible.

Consolidated both paths through a new pure helper `detectModeEntry`
that returns the new mode plus the stripped buffer value, so there is
no longer a branch where the mode character can leak into the buffer.

Fixes #662

src/components/PromptInput/inputModes.test.ts

@@ -0,0 +1,104 @@
+import { describe, expect, it } from 'bun:test'
+import {
+  detectModeEntry,
+  getModeFromInput,
+  getValueFromInput,
+  isInputModeCharacter,
+  prependModeCharacterToInput,
+} from './inputModes.js'
+
+describe('inputModes', () => {
+  describe('getModeFromInput', () => {
+    it('returns bash mode for input starting with !', () => {
+      expect(getModeFromInput('!')).toBe('bash')
+      expect(getModeFromInput('!ls')).toBe('bash')
+    })
+
+    it('returns prompt mode for non-bash input', () => {
+      expect(getModeFromInput('')).toBe('prompt')
+      expect(getModeFromInput('hello')).toBe('prompt')
+      expect(getModeFromInput(' !')).toBe('prompt')
+    })
+  })
+
+  describe('getValueFromInput', () => {
+    it('strips the leading ! when entering bash mode', () => {
+      expect(getValueFromInput('!')).toBe('')
+      expect(getValueFromInput('!ls -la')).toBe('ls -la')
+    })
+
+    it('returns input unchanged in prompt mode', () => {
+      expect(getValueFromInput('')).toBe('')
+      expect(getValueFromInput('hello')).toBe('hello')
+    })
+  })
+
+  describe('isInputModeCharacter', () => {
+    it('returns true only for the bare ! character', () => {
+      expect(isInputModeCharacter('!')).toBe(true)
+      expect(isInputModeCharacter('!ls')).toBe(false)
+      expect(isInputModeCharacter('')).toBe(false)
+    })
+  })
+
+  describe('prependModeCharacterToInput', () => {
+    it('prepends ! when mode is bash', () => {
+      expect(prependModeCharacterToInput('ls', 'bash')).toBe('!ls')
+      expect(prependModeCharacterToInput('', 'bash')).toBe('!')
+    })
+
+    it('returns input unchanged in prompt mode', () => {
+      expect(prependModeCharacterToInput('hello', 'prompt')).toBe('hello')
+    })
+  })
+
+  describe('detectModeEntry', () => {
+    // Regression for #662 — typing `!` into empty input must switch to bash
+    // mode AND yield an empty stripped buffer. Before the fix the single-char
+    // path returned without stripping, leaving `!` visible in the buffer.
+    it('strips the mode character when typing ! into empty input', () => {
+      expect(
+        detectModeEntry({ value: '!', prevInputLength: 0, cursorOffset: 0 }),
+      ).toEqual({ mode: 'bash', strippedValue: '' })
+    })
+
+    it('strips the mode character when pasting !cmd into empty input', () => {
+      expect(
+        detectModeEntry({ value: '!ls -la', prevInputLength: 0, cursorOffset: 0 }),
+      ).toEqual({ mode: 'bash', strippedValue: 'ls -la' })
+    })
+
+    it('returns null when the cursor is not at the start', () => {
+      expect(
+        detectModeEntry({ value: '!', prevInputLength: 0, cursorOffset: 1 }),
+      ).toBeNull()
+    })
+
+    it('returns null when the value does not start with !', () => {
+      expect(
+        detectModeEntry({ value: 'hello', prevInputLength: 0, cursorOffset: 0 }),
+      ).toBeNull()
+    })
+
+    it('returns null when typing ! after existing text', () => {
+      // value="ab!" with prevInputLength=2 is a single-char insertion but does
+      // not start with ! — getModeFromInput returns 'prompt'.
+      expect(
+        detectModeEntry({ value: 'ab!', prevInputLength: 2, cursorOffset: 0 }),
+      ).toBeNull()
+    })
+
+    it('returns null when prepending ! to non-empty existing text', () => {
+      // Single-char insertion at start that produces "!ab" from "ab" — value
+      // length is 3, prevInputLength is 2, so isSingleCharInsertion is true
+      // and isMultiCharIntoEmpty is false. We accept the mode change here so
+      // that typing ! at the start of existing text still toggles mode.
+      const result = detectModeEntry({
+        value: '!ab',
+        prevInputLength: 2,
+        cursorOffset: 0,
+      })
+      expect(result).toEqual({ mode: 'bash', strippedValue: 'ab' })
+    })
+  })
+})