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::`

---
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* #15953
* __->__ #15944

codex-rs/tools/README.md

@@ -9,8 +9,9 @@ schema primitives that no longer need to live in `core/src/tools/spec.rs`:
 
 - `JsonSchema`
 - `AdditionalProperties`
+- `ParsedToolDefinition`
 - `parse_tool_input_schema()`
-- `ParsedMcpTool`
+- `parse_dynamic_tool()`
 - `parse_mcp_tool()`
 - `mcp_call_tool_result_output_schema()`