fix(input): preserve file paths in the message box instead of replacing them with file contents

[mdev34-lab]

Problem

When a file path is entered/inserted into the message box, AlphaCode can replace the path with the file's contents. This destroys the original path in the user's message and makes it impossible to send the literal path as intended.

Expected behavior

File paths entered or inserted into the message box should remain paths in the message text.

If file-content attachment/expansion is an intentional feature, it should require an explicit user action and must not silently replace the path itself.

Acceptance criteria

• Typing or pasting a file path into the message box preserves the path verbatim.
• File-path detection must not mutate ordinary message text into file contents implicitly.
• Any intentional file-content insertion remains available through an explicit mechanism.
• Add a regression test covering a valid local file path and asserting that the message text still contains the path rather than the file contents.