codex-tools: extract dynamic tool adapters (#15944)

## Why

`codex-tools` already owned the shared JSON schema parser and the MCP
tool schema adapter, but `core/src/tools/spec.rs` still parsed dynamic
tools directly.

That left the tool-schema boundary split in two different ways:

- MCP tools flowed through `codex-tools`, while dynamic tools were still
parsed in `codex-core`
- the extracted dynamic-tool path initially introduced a
dynamic-specific parsed shape even though `codex-tools` already had very
similar MCP adapter output

This change finishes that extraction boundary in one step. `codex-core`
still owns `ResponsesApiTool` assembly, but both MCP tools and dynamic
tools now enter that layer through `codex-tools` using the same parsed
tool-definition shape.

## What changed

- added `tools/src/dynamic_tool.rs` and sibling
`tools/src/dynamic_tool_tests.rs`
- introduced `parse_dynamic_tool()` in `codex-tools` and switched
`core/src/tools/spec.rs` to use it for dynamic tools
- added `tools/src/parsed_tool_definition.rs` so both MCP and dynamic
adapters return the same `ParsedToolDefinition`
- updated `core/src/tools/spec.rs` to build `ResponsesApiTool` through a
shared local adapter helper instead of separate MCP and dynamic assembly
paths
- expanded `core/src/tools/spec_tests.rs` so the dynamic-tool adapter
test asserts the full converted `ResponsesApiTool`, including
`defer_loading`
- updated `codex-rs/tools/README.md` to reflect the shared parsed
tool-definition boundary

## Test plan

- `cargo test -p codex-tools`
- `cargo test -p codex-core --lib tools::spec::`

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/15944).
* #15953
* __->__ #15944

codex-rs/tools/src/mcp_tool_tests.rs

@@ -1,7 +1,7 @@
-use super::ParsedMcpTool;
 use super::mcp_call_tool_result_output_schema;
 use super::parse_mcp_tool;
 use crate::JsonSchema;
+use crate::ParsedToolDefinition;
 use pretty_assertions::assert_eq;
 use std::collections::BTreeMap;
 
@@ -31,14 +31,15 @@ fn parse_mcp_tool_inserts_empty_properties() {
 
     assert_eq!(
         parse_mcp_tool(&tool).expect("parse MCP tool"),
-        ParsedMcpTool {
+        ParsedToolDefinition {
             description: "No properties".to_string(),
             input_schema: JsonSchema::Object {
                 properties: BTreeMap::new(),
                 required: None,
                 additional_properties: None,
             },
-            output_schema: mcp_call_tool_result_output_schema(serde_json::json!({})),
+            output_schema: Some(mcp_call_tool_result_output_schema(serde_json::json!({}))),
+            defer_loading: false,
         }
     );
 }
@@ -67,14 +68,14 @@ fn parse_mcp_tool_preserves_top_level_output_schema() {
 
     assert_eq!(
         parse_mcp_tool(&tool).expect("parse MCP tool"),
-        ParsedMcpTool {
+        ParsedToolDefinition {
             description: "Has output schema".to_string(),
             input_schema: JsonSchema::Object {
                 properties: BTreeMap::new(),
                 required: None,
                 additional_properties: None,
             },
-            output_schema: mcp_call_tool_result_output_schema(serde_json::json!({
+            output_schema: Some(mcp_call_tool_result_output_schema(serde_json::json!({
                 "properties": {
                     "result": {
                         "properties": {
@@ -83,7 +84,8 @@ fn parse_mcp_tool_preserves_top_level_output_schema() {
                     }
                 },
                 "required": ["result"]
-            })),
+            }))),
+            defer_loading: false,
         }
     );
 }
@@ -105,16 +107,17 @@ fn parse_mcp_tool_preserves_output_schema_without_inferred_type() {
 
     assert_eq!(
         parse_mcp_tool(&tool).expect("parse MCP tool"),
-        ParsedMcpTool {
+        ParsedToolDefinition {
             description: "Has enum output schema".to_string(),
             input_schema: JsonSchema::Object {
                 properties: BTreeMap::new(),
                 required: None,
                 additional_properties: None,
             },
-            output_schema: mcp_call_tool_result_output_schema(serde_json::json!({
+            output_schema: Some(mcp_call_tool_result_output_schema(serde_json::json!({
                 "enum": ["ok", "error"]
-            })),
+            }))),
+            defer_loading: false,
         }
     );
 }