Extract tool config into codex-tools (#16379)

## Why

`codex-core` already owns too much of the tool stack, and `AGENTS.md`
explicitly pushes us to move shared code out of `codex-core` instead of
letting it keep growing. This PR takes the next incremental step in
moving `core/src/tools` toward `codex-rs/tools` by extracting
low-coupling tool configuration and image-detail gating logic into
`codex-tools`.

That gives later extraction work a cleaner boundary to build on without
trying to move the entire tools subtree in one shot.

## What changed

- moved `ToolsConfig`, `ToolsConfigParams`, shell backend config, and
unified-exec session selection from `core/src/tools/spec.rs` into
`codex-tools`
- moved original image-detail gating and normalization into
`codex-tools`
- updated `codex-core` to consume the new `codex-tools` exports and pass
a rendered agent-type description instead of raw role config
- kept `codex-rs/tools/src/lib.rs` exports-only, with extracted unit
tests living in sibling `*_tests.rs` modules

## Testing

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

codex-rs/core/src/codex.rs

@@ -944,7 +944,9 @@ impl TurnContext {
         .with_unified_exec_shell_mode(self.tools_config.unified_exec_shell_mode.clone())
         .with_web_search_config(self.tools_config.web_search_config.clone())
         .with_allow_login_shell(self.tools_config.allow_login_shell)
-        .with_agent_roles(config.agent_roles.clone());
+        .with_agent_type_description(crate::agent::role::spawn_tool_spec::build(
+            &config.agent_roles,
+        ));
 
         Self {
             sub_id: self.sub_id.clone(),
@@ -1396,13 +1398,15 @@ impl Session {
             windows_sandbox_level: session_configuration.windows_sandbox_level,
         })
         .with_unified_exec_shell_mode_for_session(
-            user_shell,
+            crate::tools::spec::tool_user_shell_type(user_shell),
             shell_zsh_path,
             main_execve_wrapper_exe,
         )
         .with_web_search_config(per_turn_config.web_search_config.clone())
         .with_allow_login_shell(per_turn_config.permissions.allow_login_shell)
-        .with_agent_roles(per_turn_config.agent_roles.clone());
+        .with_agent_type_description(crate::agent::role::spawn_tool_spec::build(
+            &per_turn_config.agent_roles,
+        ));
 
         let cwd = session_configuration.cwd.clone();
 
@@ -5468,13 +5472,15 @@ async fn spawn_review_thread(
         windows_sandbox_level: parent_turn_context.windows_sandbox_level,
     })
     .with_unified_exec_shell_mode_for_session(
-        sess.services.user_shell.as_ref(),
+        crate::tools::spec::tool_user_shell_type(sess.services.user_shell.as_ref()),
         sess.services.shell_zsh_path.as_ref(),
         sess.services.main_execve_wrapper_exe.as_ref(),
     )
     .with_web_search_config(/*web_search_config*/ None)
     .with_allow_login_shell(config.permissions.allow_login_shell)
-    .with_agent_roles(config.agent_roles.clone());
+    .with_agent_type_description(crate::agent::role::spawn_tool_spec::build(
+        &config.agent_roles,
+    ));
 
     let review_prompt = resolved.prompt.clone();
     let provider = parent_turn_context.provider.clone();