Consolidate chat attachment copying

[Roman Lutz (romanlutz)]

ChatWindow duplicated the same message-to-input attachment loop in its direct copy and copy-to-new-conversation actions, making the two paths easy to change inconsistently.

This extracts a shared, component-local copy operation for the identical success behavior. It preserves non-file attachment ordering and metadata, keeps the delayed new-conversation update unchanged, and leaves the distinct text-only creation-failure fallback separate.

Focused tests cover both shared call paths, file filtering, attachment order and metadata preservation, and the failure fallback.

Validation:

• Focused ChatWindow Jest tests
• ESLint on changed files
• Frontend TypeScript type-check
• git diff --check

[hannahwestra25]

nit:

Suggested change

	const inputBox = inputBoxRef.current
	if (!inputBox) { return }
	if (message.content) {
	inputBox.setText(message.content)
	}
	if (!message.attachments) { return }
	for (const attachment of message.attachments) {
	if (attachment.type !== 'file') {
	inputBox.addAttachment(attachment)
	}
	}
	const inputBox = inputBoxRef.current
	if (!inputBox) return
	if (message.content) {
	inputBox.setText(message.content)
	}
	for(const attachment of message.attachments ?? []) {
	if (attachment.type !== 'file') {
	inputBox.addAttachment(attachment)
	}
	}