[codex] accept env file refs for app uploads

[lt-oai]

What

• Allows openai/fileParams arguments to use env://current/<relative-path> file references.
• Updates model-visible file-param guidance to mention both local paths and current-env refs.

Why

• Lets Code Mode materialize files with the new harness primitive and pass those refs into existing Apps file-param tools.
• Keeps backward compatibility with existing local-path upload behavior.

How

• Extends broker input resolution to handle env://current refs.
• Rejects unsupported env://... environments, empty paths, absolute env paths, and cwd escapes.
• Uploads resolved env refs through the same OpenAI file storage path as local paths.

Validation

• cargo test -p codex-core mcp_openai_file -- --nocapture
• cargo test -p codex-mcp tools -- --nocapture
• cargo test -p codex-core --test all codex_apps_file_params_upload_local_paths_before_mcp_tool_call -- --nocapture