codex-tools: extract configured tool specs (#16129)

## Why

This continues the `codex-tools` migration by moving another passive
tool-spec layer out of `codex-core`.

After `ToolSpec` moved into `codex-tools`, `codex-core` still owned
`ConfiguredToolSpec` and `create_tools_json_for_responses_api()`. Both
are data-model and serialization helpers rather than runtime
orchestration, so keeping them in `core/src/tools/registry.rs` and
`core/src/tools/spec.rs` left passive tool-definition code coupled to
`codex-core` longer than necessary.

## What changed

- moved `ConfiguredToolSpec` into `codex-rs/tools/src/tool_spec.rs`
- moved `create_tools_json_for_responses_api()` into
`codex-rs/tools/src/tool_spec.rs`
- re-exported the new surface from `codex-rs/tools/src/lib.rs`, which
remains exports-only
- updated `core/src/client.rs`, `core/src/tools/registry.rs`, and
`core/src/tools/router.rs` to consume the extracted types and serializer
from `codex-tools`
- moved the tool-list serialization test into
`codex-rs/tools/src/tool_spec_tests.rs`
- added focused unit coverage for `ConfiguredToolSpec::name()`
- simplified `core/src/tools/spec_tests.rs` to use the extracted
`ConfiguredToolSpec::name()` directly and removed the now-redundant
local `tool_name()` helper
- updated `codex-rs/tools/README.md` so the crate boundary reflects the
newly extracted tool-spec wrapper and serialization helper

## Test plan

- `cargo test -p codex-tools`
- `CARGO_TARGET_DIR=/tmp/codex-core-configured-spec cargo test -p
codex-core --lib tools::spec::`
- `CARGO_TARGET_DIR=/tmp/codex-core-configured-spec cargo test -p
codex-core --lib client::`
- `just fix -p codex-tools -p codex-core`
- `just argument-comment-lint`

## References

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

codex-rs/tools/README.md

@@ -12,6 +12,7 @@ schema and Responses API tool primitives that no longer need to live in
 - `AdditionalProperties`
 - `ToolDefinition`
 - `ToolSpec`
+- `ConfiguredToolSpec`
 - `ResponsesApiTool`
 - `FreeformTool`
 - `FreeformToolFormat`
@@ -23,6 +24,7 @@ schema and Responses API tool primitives that no longer need to live in
 - `parse_tool_input_schema()`
 - `parse_dynamic_tool()`
 - `parse_mcp_tool()`
+- `create_tools_json_for_responses_api()`
 - `mcp_call_tool_result_output_schema()`
 - `tool_definition_to_responses_api_tool()`
 - `dynamic_tool_to_responses_api_tool()`