POC: Expose gated Code Mode files tools

[lt-oai]

Why

Code Mode needs a concrete native surface for the proposed file platform actions: materialize a file into the workspace, copy between refs, and export a fileParam-compatible data URI at tool-call time.

What

• Adds a files namespace gated by experimental_supported_tools = ["code_mode_files"].
• Implements files.materialize, files.copy, and files.export_for_tool over the file broker.
• Adds an active provider registry: configured broker routes are checked against live MCP inventory from mcp_connection_manager.list_all_tools().
• Lets broker routes delegate to installed MCP tools internally, while those same MCP tools may still be visible to the model directly.
• Keeps openai/fileParams as compatibility metadata, not the source of truth. Explicit broker routes/adapters decide semantics.
• Keeps the POC limited to env://current/...; provider adapters can fill in Library, connector, and remote env behavior behind the broker.

Testing

• cargo test -p codex-core files
• cargo test -p codex-core file_broker
• just fmt