feat: add Codex Apps sediment file remapping (#15197)

## Summary
- bridge Codex Apps tools that declare `_meta["openai/fileParams"]`
through the OpenAI file upload flow
- mask those file params in model-visible tool schemas so the model
provides absolute local file paths instead of raw file payload objects
- rewrite those local file path arguments client-side into
`ProvidedFilePayload`-shaped objects before the normal MCP tool call

## Details
- applies to scalar and array file params declared in
`openai/fileParams`
- Codex uploads local files directly to the backend and uses the
uploaded file metadata to build the MCP tool arguments locally
- this PR is input-only

## Verification
- `just fmt`
- `cargo test -p codex-core mcp_tool_call -- --nocapture`

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-api/src/lib.rs

@@ -3,6 +3,7 @@ pub(crate) mod auth;
 pub(crate) mod common;
 pub(crate) mod endpoint;
 pub(crate) mod error;
+pub(crate) mod files;
 pub(crate) mod provider;
 pub(crate) mod rate_limits;
 pub(crate) mod requests;
@@ -52,6 +53,7 @@ pub use crate::endpoint::ResponsesWebsocketClient;
 pub use crate::endpoint::ResponsesWebsocketConnection;
 pub use crate::endpoint::session_update_session_json;
 pub use crate::error::ApiError;
+pub use crate::files::upload_local_file;
 pub use crate::provider::Provider;
 pub use crate::provider::RetryConfig;
 pub use crate::provider::is_azure_responses_wire_base_url;