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/Cargo.lock

@@ -2518,7 +2518,6 @@ dependencies = [
  "codex-terminal-detection",
  "codex-test-binary-support",
  "codex-thread-store",
- "codex-tool-api",
  "codex-tools",
  "codex-utils-absolute-path",
  "codex-utils-cache",
@@ -2820,7 +2819,7 @@ name = "codex-extension-api"
 version = "0.0.0"
 dependencies = [
  "codex-protocol",
- "codex-tool-api",
+ "codex-tools",
 ]
 
 [[package]]
@@ -3710,15 +3709,6 @@ dependencies = [
  "uuid",
 ]
 
-[[package]]
-name = "codex-tool-api"
-version = "0.0.0"
-dependencies = [
- "pretty_assertions",
- "serde_json",
- "thiserror 2.0.18",
-]
-
 [[package]]
 name = "codex-tools"
 version = "0.0.0"