feat: wire extension tool bundles into core (#22147)

## Why

This is the next narrow step toward moving concrete tool families out of
core. After #22138 introduced `codex-tool-api`, we still needed a real
end-to-end seam that lets an extension own an executable tool definition
once and have core install it without the temporary `extension-api`
wrapper or a dependency on `codex-tools`.

`codex-tool-api` is the small extension-facing execution contract, while
`codex-tools` still has a different job: host-side shared tool metadata
and planning logic that is not “run this contributed tool”, like spec
shaping, namespaces, discovery, code-mode augmentation, and
MCP/dynamic-to-Responses API conversion

## What changed

- Moved the shared leaf tool-spec and JSON Schema types into
`codex-tool-api`, so the executable contract now lives with
[`ToolBundle`](https://github.com/openai/codex/blob/c538758095337d4fe0a52a172363ccede4066bda/codex-rs/tool-api/src/bundle.rs#L19-L70).
- Replaced the temporary extension-side tool wrapper with direct
`ToolBundle` use in `codex-extension-api`.
- Taught core to collect contributed bundles, include them in spec
planning, register them through
[`ToolRegistryBuilder::register_tool_bundle`](https://github.com/openai/codex/blob/c538758095337d4fe0a52a172363ccede4066bda/codex-rs/core/src/tools/registry.rs#L653-L667),
and dispatch them through the existing router/runtime path.
- Added focused coverage for contributed tools becoming model-visible
and dispatchable, plus spec-planning coverage for contributed function
and freeform tools.

## Verification

- Added `extension_tool_bundles_are_model_visible_and_dispatchable` in
`core/src/tools/router_tests.rs`.
- Added spec-plan coverage in `core/src/tools/spec_plan_tests.rs` for
contributed extension bundles.

## Related

- Follow-up to #22138

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

@@ -29,6 +29,7 @@ use crate::tools::parallel::ToolCallRuntime;
 use crate::tools::router::ToolCall;
 use crate::tools::router::ToolCallSource;
 use crate::tools::router::ToolRouterParams;
+use crate::tools::router::extension_tool_bundles;
 use crate::unified_exec::resolve_max_tokens;
 use codex_features::Feature;
 use codex_tools::ToolName;
@@ -285,6 +286,7 @@ async fn build_nested_router(exec: &ExecContext) -> ToolRouter {
             unavailable_called_tools: Vec::new(),
             parallel_mcp_server_names,
             discoverable_tools: None,
+            extension_tool_bundles: extension_tool_bundles(exec.session.as_ref()),
             dynamic_tools: exec.turn.dynamic_tools.as_slice(),
         },
     )