Refactor extension tools onto shared ToolExecutor (#22369)

## Why

Extension tools were split across two public runtime contracts:
`codex-tool-api` exposed `ToolBundle` plus its own call/spec/error
types, while core native tools used `codex_tools::ToolExecutor`. That
made contributed tool specs and execution behavior easy to drift apart
and added another crate boundary for what should be one executable-tool
seam.

This PR makes `ToolExecutor` the single runtime contract and keeps
extension-specific pinning in `codex-extension-api`.

## Remaining todo

https://github.com/openai/codex/pull/22369/changes#diff-b935ea8245c3ce568a30cff660175fa6390b66b872ae409e1e2e965738250741R5
Either generic `Invocation` or sub-extract the `ToolCall` and clean
`ToolInvocation`

## What changed

- Removed the `codex-tool-api` workspace crate and its dependencies from
core and `codex-extension-api`.
- Made `codex_tools::ToolExecutor` object-safe with `async_trait` so
extension contributors can return a dyn executor.
- Added the extension-facing aliases under
`ext/extension-api/src/contributors/tools.rs`, including
`ExtensionToolExecutor = dyn ToolExecutor<ToolCall, Output =
ExtensionToolOutput>`.
- Changed `ToolContributor::tools` to return extension executors
directly instead of `ToolBundle`s.
- Updated core’s extension tool handler/registry/router path to adapt
those extension executors into the existing native `ToolInvocation`
runtime path.
- Added focused coverage for extension tools being registered,
model-visible, dispatchable, and not replacing built-in tools.

## Verification

- `cargo test -p codex-tools`
- `cargo test -p codex-extension-api`

codex-rs/core/src/tools/spec_tests.rs

@@ -275,7 +275,7 @@ fn build_specs(
         mcp_tools,
         deferred_mcp_tools,
         /*discoverable_tools*/ None,
-        /*extension_tool_bundles*/ &[],
+        /*extension_tool_executors*/ &[],
         dynamic_tools,
     )
 }
@@ -335,7 +335,7 @@ async fn assert_model_tools(
             mcp_tools: None,
             deferred_mcp_tools: None,
             discoverable_tools: None,
-            extension_tool_bundles: Vec::new(),
+            extension_tool_executors: Vec::new(),
             dynamic_tools: &[],
         },
     );
@@ -804,7 +804,7 @@ async fn request_plugin_install_requires_apps_and_plugins_features() {
             /*mcp_tools*/ None,
             /*deferred_mcp_tools*/ None,
             discoverable_tools.clone(),
-            /*extension_tool_bundles*/ &[],
+            /*extension_tool_executors*/ &[],
             &[],
         )
         .build();