Fix tabs being printed in cmd.exe prompts (#16273)

A late change in #16105 wrapped `_buffer` into a class to better track
its dirty state, but I failed to notice that in this one instance we
intentionally manipulated `_buffer` without marking it as dirty.
This fixes the issue by adding a call to `MarkAsClean()`.

This changeset also adds the test instructions from #15783 as a
document to this repository. I've extended the list with two
bugs we've found in the implementation since then.

## Validation Steps Performed
* In cmd.exe, with an empty prompt in an empty directory:
  Pressing tab produces an audible bing and prints no text ✅

doc/COOKED_READ_DATA.md

@@ -0,0 +1,90 @@
+# COOKED_READ_DATA, aka conhost's readline implementation
+
+## Test instructions
+
+All of the following ✅ marks must be fulfilled during manual testing:
+* ASCII input
+* Chinese input (中文維基百科) ❔
+  * Resizing the window properly wraps/unwraps wide glyphs ❌
+    Broken due to `TextBuffer::Reflow` bugs
+* Surrogate pair input (🙂) ❔
+  * Resizing the window properly wraps/unwraps surrogate pairs ❌
+    Broken due to `TextBuffer::Reflow` bugs
+* In cmd.exe
+  * Create 2 file: "a😊b.txt" and "a😟b.txt"
+  * Press tab: Autocomplete to "a😊b.txt" ✅
+  * Navigate the cursor right past the "a"
+  * Press tab twice: Autocomplete to "a😟b.txt" ✅
+* Backspace deletes preceding glyphs ✅
+* Ctrl+Backspace deletes preceding words ✅
+* Escape clears input ✅
+* Home navigates to start ✅
+* Ctrl+Home deletes text between cursor and start ✅
+* End navigates to end ✅
+* Ctrl+End deletes text between cursor and end ✅
+* Left navigates over previous code points ✅
+* Ctrl+Left navigates to previous word-starts ✅
+* Right and F1 navigate over next code points ✅
+  * Pressing right at the end of input copies characters
+    from the previous command ✅
+* Ctrl+Right navigates to next word-ends ✅
+* Insert toggles overwrite mode ✅
+* Delete deletes next code point ✅
+* Up and F5 cycle through history ✅
+  * Doesn't crash with no history ✅
+  * Stops at first entry ✅
+* Down cycles through history ✅
+  * Doesn't crash with no history ✅
+  * Stops at last entry ✅
+* PageUp retrieves the oldest command ✅
+* PageDown retrieves the newest command ✅
+* F2 starts "copy to char" prompt ✅
+  * Escape dismisses prompt ✅
+  * Typing a character copies text from the previous command up
+    until that character into the current buffer (acts identical
+    to F3, but with automatic character search) ✅
+* F3 copies the previous command into the current buffer,
+  starting at the current cursor position,
+  for as many characters as possible ✅
+  * Doesn't erase trailing text if the current buffer
+    is longer than the previous command ✅
+  * Puts the cursor at the end of the copied text ✅
+* F4 starts "copy from char" prompt ✅
+  * Escape dismisses prompt ✅
+  * Erases text between the current cursor position and the
+    first instance of a given char (but not including it) ✅
+* F6 inserts Ctrl+Z ✅
+* F7 without modifiers starts "command list" prompt ✅
+  * Escape dismisses prompt ✅
+  * Minimum size of 40x10 characters ✅
+  * Width expands to fit the widest history command ✅
+  * Height expands up to 20 rows with longer histories ✅
+  * F9 starts "command number" prompt ✅
+  * Left/Right paste replace the buffer with the given command ✅
+    * And put cursor at the end of the buffer ✅
+  * Up/Down navigate selection through history ✅
+    * Stops at start/end with <10 entries ✅
+    * Stops at start/end with >20 entries ✅
+    * Wide text rendering during pagination with >20 entries ✅
+  * Shift+Up/Down moves history items around ✅
+  * Home navigates to first entry ✅
+  * End navigates to last entry ✅
+  * PageUp navigates by 20 items at a time or to first ✅
+  * PageDown navigates by 20 items at a time or to last ✅
+* Alt+F7 clears command history ✅
+* F8 cycles through commands that start with the same text as
+  the current buffer up until the current cursor position ✅
+  * Doesn't crash with no history ✅
+* F9 starts "command number" prompt ✅
+  * Escape dismisses prompt ✅
+  * Ignores non-ASCII-decimal characters ✅
+  * Allows entering between 1 and 5 digits ✅
+  * Pressing Enter fetches the given command from the history ✅
+* Alt+F10 clears doskey aliases ✅
+* In cmd.exe, with an empty prompt in an empty directory:
+  Pressing tab produces an audible bing and prints no text ✅
+* When Narrator is enabled, in cmd.exe:
+  * Typing individual characters announces only
+    exactly each character that is being typed ✅
+  * Backspacing at the end of a prompt announces
+    only exactly each deleted character ✅

src/host/readDataCooked.cpp

@@ -436,14 +436,17 @@ void COOKED_READ_DATA::_handleChar(wchar_t wch, const DWORD modifiers)
 
     if (_ctrlWakeupMask != 0 && wch < L' ' && (_ctrlWakeupMask & (1 << wch)))
     {
-        _flushBuffer();
-
         // The old implementation (all the way since the 90s) overwrote the character at the current cursor position with the given wch.
         // But simultaneously it incremented the buffer length, which would have only worked if it was written at the end of the buffer.
         // Press tab past the "f" in the string "foo" and you'd get "f\to " (a trailing whitespace; the initial contents of the buffer back then).
         // It's unclear whether the original intention was to write at the end of the buffer at all times or to implement an insert mode.
         // I went with insert mode.
+        //
+        // It is important that we don't actually print that character out though, as it's only for the calling application to see.
+        // That's why we flush the contents before the insertion and then ensure that the _flushBuffer() call in Read() exits early.
+        _flushBuffer();
         _buffer.Replace(_buffer.GetCursorPosition(), 0, &wch, 1);
+        _buffer.MarkAsClean();
 
         _controlKeyState = modifiers;
         _transitionState(State::DoneWithWakeupMask);