bug: dragging in screenshot always results in initial "Tool validation failed" message

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

macOS Sequoia 15.7.1

### Expected behaviour

I expected that dragging in a screenshot would result in a successful upload with no error messages.

### Actual behaviour

You submit any prompt with a screenshot with or without additional text and you end up getting an initial message of tool validation failed. It eventually does succeed as it tries again utilizing the fs_read tool but the initial tool validation failure message is unexpected.

### Steps to reproduce

Submit any prompt or an empty prompt and drag in a screenshot from a folder into the terminal. Then submit and see the agent response.

### Environment

```yaml
[q-details]
version = "1.19.3"
hash = "84b82da54996570ac369ec69911e29b524099761"
date = "2025-10-29T21:11:53.813462Z (2d ago)"
variant = "full"

[system-info]
os = "macOS 15.7.1 (24G231)"
chip = "Apple M3 Pro"
total-cores = 12
memory = "36.00 GB"

[environment]
cwd = "/Users/USER"
cli-path = "/Users/USER"
os = "Mac"
shell-path = "/Applications/Amazon Q.app/Contents/MacOS/qchat"
terminal = "macOS"
install-method = "unknown"

[env-vars]
PATH = "/Users/USER/.local/share/mise/installs/node/18.20.2/bin:/Users/USER/.local/share/mise/installs/python/3.12.12/bin:/Users/USER/.local/share/mise/installs/python/3.11.14/bin:/Users/USER/.local/share/mise/installs/python/3.10.19/bin:/Users/USER/.local/share/mise/installs/python/3.9.24/bin:/Users/USER/.local/share/mise/installs/python/3.8.20/bin:/opt/homebrew/bin:/opt/homebrew/sbin:/Users/USER/.toolbox/bin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Users/USER/.cargo/bin:/Users/USER/.local/bin:/Applications/Visual Studio Code.app/Contents/Resources/app/bin"
QTERM_SESSION_ID = "c06a1ea4e8534626af8b6d698955bea5"
Q_SET_PARENT_CHECK = "1"
Q_TERM = "1.19.3"
SHELL = "/bin/zsh"
TERM = "xterm-256color"
__CFBundleIdentifier = "com.apple.Terminal"
```