codex-tools: extract tool spec models (#16047)

## Why

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

After `ResponsesApiTool` and the lower-level schema adapters moved into
`codex-tools`, `core/src/client_common.rs` was still owning `ToolSpec`
and the web-search request wire types even though they are serialized
data models rather than runtime orchestration. Keeping those types in
`codex-core` makes the crate boundary look smaller than it really is and
leaves non-runtime tool-shape code coupled to core.

## What changed

- moved `ToolSpec`, `ResponsesApiWebSearchFilters`, and
`ResponsesApiWebSearchUserLocation` into
`codex-rs/tools/src/tool_spec.rs`
- added focused unit tests in `codex-rs/tools/src/tool_spec_tests.rs`
for:
  - `ToolSpec::name()`
  - web-search config conversions
  - `ToolSpec` serialization for `web_search` and `tool_search`
- kept `codex-rs/tools/src/lib.rs` exports-only by re-exporting the new
module from `lib.rs`
- reduced `core/src/client_common.rs` to a compatibility shim that
re-exports the extracted tool-spec types for current core call sites
- updated `core/src/tools/spec_tests.rs` to consume the extracted
web-search types directly from `codex-tools`
- updated `codex-rs/tools/README.md` so the crate contract reflects that
`codex-tools` now owns the passive tool-spec request models in addition
to the lower-level Responses API structs

## Test plan

- `cargo test -p codex-tools`
- `cargo test -p codex-core --lib tools::spec::`
- `cargo test -p codex-core --lib client_common::`
- `just fix -p codex-tools -p codex-core`
- `just argument-comment-lint`

## References

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

codex-rs/tools/src/lib.rs

@@ -6,6 +6,7 @@ mod json_schema;
 mod mcp_tool;
 mod responses_api;
 mod tool_definition;
+mod tool_spec;
 
 pub use dynamic_tool::parse_dynamic_tool;
 pub use json_schema::AdditionalProperties;
@@ -24,3 +25,6 @@ pub use responses_api::mcp_tool_to_deferred_responses_api_tool;
 pub use responses_api::mcp_tool_to_responses_api_tool;
 pub use responses_api::tool_definition_to_responses_api_tool;
 pub use tool_definition::ToolDefinition;
+pub use tool_spec::ResponsesApiWebSearchFilters;
+pub use tool_spec::ResponsesApiWebSearchUserLocation;
+pub use tool_spec::ToolSpec;