codex-tools: extract MCP schema adapters (#15928)

## Why

`codex-tools` already owns the shared tool input schema model and parser
from the first extraction step, but `core/src/tools/spec.rs` still owned
the MCP-specific adapter that normalizes `rmcp::model::Tool` schemas and
wraps `structuredContent` into the call result output schema.

Keeping that adapter in `codex-core` means the reusable MCP schema path
is still split across crates, and the unit tests for that logic stay
anchored in `codex-core` even though the runtime orchestration does not
need to move yet.

This change takes the next small step by moving the reusable MCP schema
adapter into `codex-tools` while leaving `ResponsesApiTool` assembly in
`codex-core`.

## What changed

- added `tools/src/mcp_tool.rs` and sibling
`tools/src/mcp_tool_tests.rs`
- introduced `ParsedMcpTool`, `parse_mcp_tool()`, and
`mcp_call_tool_result_output_schema()` in `codex-tools`
- updated `core/src/tools/spec.rs` to consume parsed MCP tool parts from
`codex-tools`
- removed the now-redundant MCP schema unit tests from
`core/src/tools/spec_tests.rs`
- expanded `codex-rs/tools/README.md` to describe this second migration
step

## Test plan

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

codex-rs/tools/src/lib.rs

@@ -1,7 +1,11 @@
 //! Shared tool-schema parsing primitives that can live outside `codex-core`.
 
 mod json_schema;
+mod mcp_tool;
 
 pub use json_schema::AdditionalProperties;
 pub use json_schema::JsonSchema;
 pub use json_schema::parse_tool_input_schema;
+pub use mcp_tool::ParsedMcpTool;
+pub use mcp_tool::mcp_call_tool_result_output_schema;
+pub use mcp_tool::parse_mcp_tool;