Extract tool registry planning into codex-tools (#16513)

## Why
This is a larger step in the `codex-core` -> `codex-tools` migration
called out in `AGENTS.md`.

`codex-rs/core/src/tools/spec.rs` had become mostly pure tool-spec
assembly plus handler registration. That made it hard to move more of
the tool-definition layer into `codex-tools`, because the runtime
binding and the crate-independent planning logic were still interleaved
in one function.

Splitting those concerns gives `codex-tools` ownership of the
declarative registry plan while keeping `codex-core` responsible for
instantiating concrete handlers.

## What Changed
- Add a `codex-tools` registry-plan layer in
`codex-rs/tools/src/tool_registry_plan.rs` and
`codex-rs/tools/src/tool_registry_plan_types.rs`.
- Move feature-gated tool-spec assembly, MCP/dynamic tool conversion,
tool-search aliases, and code-mode nested-plan expansion into
`codex-tools`.
- Keep `codex-rs/core/src/tools/spec.rs` as the core-side adapter that
maps each planned handler kind to concrete runtime handler instances.
- Update `spec_tests.rs` to import the moved `codex_tools` symbols
directly instead of relying on top-level `spec.rs` re-exports.

This is intended to be a straight refactor with no behavior change and
no new test surface.

## Verification
- `cargo test -p codex-tools`
- `cargo test -p codex-core tools::spec::tests`

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/16513).
* #16521
* __->__ #16513

codex-rs/tools/src/lib.rs

@@ -18,6 +18,8 @@ mod responses_api;
 mod tool_config;
 mod tool_definition;
 mod tool_discovery;
+mod tool_registry_plan;
+mod tool_registry_plan_types;
 mod tool_spec;
 mod tool_suggest;
 mod utility_tool;
@@ -107,6 +109,12 @@ pub use tool_discovery::collect_tool_suggest_entries;
 pub use tool_discovery::create_tool_search_tool;
 pub use tool_discovery::create_tool_suggest_tool;
 pub use tool_discovery::filter_tool_suggest_discoverable_tools_for_client;
+pub use tool_registry_plan::build_tool_registry_plan;
+pub use tool_registry_plan_types::ToolHandlerKind;
+pub use tool_registry_plan_types::ToolHandlerSpec;
+pub use tool_registry_plan_types::ToolRegistryPlan;
+pub use tool_registry_plan_types::ToolRegistryPlanAppTool;
+pub use tool_registry_plan_types::ToolRegistryPlanParams;
 pub use tool_spec::ConfiguredToolSpec;
 pub use tool_spec::ResponsesApiWebSearchFilters;
 pub use tool_spec::ResponsesApiWebSearchUserLocation;