Remove client_common tool re-exports (#16482)

## Why

`codex-rs/core/src/client_common.rs` still had a `tools` re-export
module that forwarded `codex_tools` types back into `codex-core`. After
the earlier extraction work in #16379, #16471, #16477, and #16481, that
extra layer no longer adds value.

Removing it keeps dependencies explicit: the `codex-core` modules that
actually use `ToolSpec` and related types now depend on `codex_tools`
directly instead of reaching through `client_common`.

## What Changed

- removed the `client_common::tools` re-export module from
`core/src/client_common.rs`
- updated the remaining `codex-core` consumers to import `codex_tools`
directly
- adjusted the affected test code to reference
`codex_tools::ResponsesApiTool` directly as well

This is a mechanical cleanup only. It does not change tool behavior or
runtime logic.

## Testing

- `cargo test -p codex-core client_common::tests`
- `cargo test -p codex-core tools::router::tests`
- `cargo test -p codex-core tools::context::tests`
- `cargo test -p codex-core tools::spec::tests`

codex-rs/core/src/tools/code_mode/mod.rs

@@ -14,7 +14,6 @@ use codex_protocol::models::ResponseInputItem;
 use serde_json::Value as JsonValue;
 use tokio_util::sync::CancellationToken;
 
-use crate::client_common::tools::ToolSpec;
 use crate::codex::Session;
 use crate::codex::TurnContext;
 use crate::function_tool::FunctionCallError;
@@ -28,6 +27,7 @@ use crate::tools::router::ToolCallSource;
 use crate::tools::router::ToolRouterParams;
 use crate::unified_exec::resolve_max_tokens;
 use codex_features::Feature;
+use codex_tools::ToolSpec;
 use codex_tools::tool_spec_to_code_mode_tool_definition;
 use codex_utils_output_truncation::TruncationPolicy;
 use codex_utils_output_truncation::formatted_truncate_text_content_items_with_policy;