textinput: Initial letter only shown in placeholder

**Describe the bug**
The placeholder value only shows the placeholder's first character when rendered, no matter the property's value length. This first appeared in v0.21.0

**Setup**
Please complete the following information along with version numbers, if applicable.
- OS: macOS (15.3.2)
- Shell: zsh (omz)
- Terminal Emulator: kitty (0.41.1)
- Locale: en_GB.UTF-8

**To Reproduce**
Steps to reproduce the behavior:
- Use the textinput module (example code will work for reproduction, but I found this in my project)
- Build & run the code

**Source Code**
[Official example code (https://github.com/charmbracelet/bubbletea/blob/main/examples/textinputs/main.go).
Here is a snippet showing the placeholder values being set
```go
for i := range m.inputs {
t = textinput.New()
t.Cursor.Style = cursorStyle
t.CharLimit = 32

switch i {
case 0:
t.Placeholder = "Nickname"
t.Focus()
t.PromptStyle = focusedStyle
t.TextStyle = focusedStyle
case 1:
t.Placeholder = "Email"
t.CharLimit = 64
case 2:
t.Placeholder = "Password"
t.EchoMode = textinput.EchoPassword
t.EchoCharacter = '•'
}

m.inputs[i] = t
}
```

**Expected behavior**
The placeholder should render the full value. In the example code, this should be:
- Nickname
- Email
- Password

**Screenshots**
These are with my values: They are supposed to be "Artist" & "Title".
It instead shows "A" & "T".

**Additional context**
The current fix is to revert to v0.20.0, and the bug does not appear

Start with the official textinputs example and the textinput module's rendering path, comparing behavior with the v0.20.0 and v0.21.0 versions. Done means the full placeholder values render as "Nickname", "Email", and "Password" rather than only their first characters.

Written by the indexing model from the issue text.