[core] Separate tools config from openai client (#1858)

## Summary
In an effort to make tools easier to work with and more configurable,
I'm introducing `ToolConfig` and updating `Prompt` to take in a general
list of Tools. I think this is simpler and better for a few reasons:
- We can easily assemble tools from various sources (our own harness,
mcp servers, etc.) and we can consolidate the logic for constructing the
logic in one place that is separate from serialization.
- client.rs no longer needs arbitrary config values, it just takes in a
list of tools to serialize

A hefty portion of the PR is now updating our conversion of
`mcp_types::Tool` to `OpenAITool`, but considering that @bolinfest
accurately called this out as a TODO long ago, I think it's time we
tackled it.

## Testing
- [x] Experimented locally, no changes, as expected
- [x] Added additional unit tests
- [x] Responded to rust-review

codex-rs/core/src/client_common.rs

@@ -3,12 +3,12 @@ use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use crate::error::Result;
 use crate::model_family::ModelFamily;
 use crate::models::ResponseItem;
+use crate::openai_tools::OpenAiTool;
 use crate::protocol::TokenUsage;
 use codex_apply_patch::APPLY_PATCH_TOOL_INSTRUCTIONS;
 use futures::Stream;
 use serde::Serialize;
 use std::borrow::Cow;
-use std::collections::HashMap;
 use std::pin::Pin;
 use std::task::Context;
 use std::task::Poll;
@@ -33,10 +33,9 @@ pub struct Prompt {
     /// Whether to store response on server side (disable_response_storage = !store).
     pub store: bool,
 
-    /// Additional tools sourced from external MCP servers. Note each key is
-    /// the "fully qualified" tool name (i.e., prefixed with the server name),
-    /// which should be reported to the model in place of Tool::name.
-    pub extra_tools: HashMap<String, mcp_types::Tool>,
+    /// Tools available to the model, including additional tools sourced from
+    /// external MCP servers.
+    pub tools: Vec<OpenAiTool>,
 
     /// Optional override for the built-in BASE_INSTRUCTIONS.
     pub base_instructions_override: Option<String>,