codex-tools: extract code mode tool spec adapters (#16132)

## Why

The longer-term `codex-tools` migration is to move pure tool-definition
and tool-spec plumbing out of `codex-core` while leaving session- and
runtime-coupled orchestration behind.

The remaining code-mode adapter layer in
`core/src/tools/code_mode_description.rs` was a good next extraction
seam because it only transformed `ToolSpec` values for code mode and
already delegated the low-level description rendering to
`codex-code-mode`.

## What Changed

- added `codex-rs/tools/src/code_mode.rs` with
`augment_tool_spec_for_code_mode()` and
`tool_spec_to_code_mode_tool_definition()`
- added focused unit coverage in `codex-rs/tools/src/code_mode_tests.rs`
- rewired `core/src/tools/spec.rs` and `core/src/tools/code_mode/mod.rs`
to use the extracted adapters from `codex-tools`
- removed the old `core/src/tools/code_mode_description.rs` shim and its
test file from `codex-core`
- added the `codex-code-mode` dependency to `codex-tools`, updated
`Cargo.lock`, and refreshed the `codex-tools` README to reflect the
expanded boundary

## Test Plan

- `cargo test -p codex-tools`
- `CARGO_TARGET_DIR=/tmp/codex-core-code-mode-adapters cargo test -p
codex-core --lib tools::spec::`
- `CARGO_TARGET_DIR=/tmp/codex-core-code-mode-adapters cargo test -p
codex-core --lib tools::code_mode::`
- `just bazel-lock-update`
- `just bazel-lock-check`
- `just argument-comment-lint`

## References

- #15923
- #15928
- #15944
- #15953
- #16031
- #16047
- #16129

codex-rs/tools/src/lib.rs

@@ -1,6 +1,7 @@
 //! Shared tool definitions and Responses API tool primitives that can live
 //! outside `codex-core`.
 
+mod code_mode;
 mod dynamic_tool;
 mod json_schema;
 mod mcp_tool;
@@ -8,6 +9,8 @@ mod responses_api;
 mod tool_definition;
 mod tool_spec;
 
+pub use code_mode::augment_tool_spec_for_code_mode;
+pub use code_mode::tool_spec_to_code_mode_tool_definition;
 pub use dynamic_tool::parse_dynamic_tool;
 pub use json_schema::AdditionalProperties;
 pub use json_schema::JsonSchema;