[codex] add sep2356 file input encoder

[lt-oai]

What

• Adds a broker-side SEP-2356 data URI encoder for local paths and env://current file refs.
• Covers RFC 2397 name= percent encoding, MIME selection, base64 encoding, and cwd escape rejection.

Why

• Prepares the broker for MCP SEP-2356 file-input compatibility while the upstream Rust MCP type does not yet expose Tool.inputFiles.
• Keeps the compatibility shape close to the existing file broker so future wiring can reuse the same path resolution checks.

How

• Encodes files as data:<media-type>;name=<filename>;base64,<data>.
• Reuses the broker's local/env file-ref resolver.
• Leaves the encoder intentionally unwired until the MCP SDK exposes the accepted inputFiles descriptor field.

Validation

• cargo test -p codex-core mcp_openai_file -- --nocapture