Missing nonbinary file contents are silently emitted as an empty complete file

[adamtheturtle]

Problem

A file object with no contents and no binary:true flag becomes contents:"" without an error. The caller cannot distinguish a genuinely empty source file from an upstream schema omission.

Evidence

assemblePadCodeFiles uses contents ?? "" and only adds an error when rawContents is present and non-null.

Acceptance criteria

• Mark missing contents on nonbinary files as a schema error/incomplete payload.

Discovered during a systematic audit of CoderPadMCP against its tool schemas and the current CoderPad API contracts.