core: prevent external tool shadowing and stabilize tool_search discovery

Per docs/contributing.md, external code contributions are invitation-only, so I am opening an issue first with analysis plus a ready branch.

Summary

I found two related problems in codex-rs/core while evaluating Codex CLI tool discovery behavior:

1. External MCP/dynamic/app-alias tools can register names that collide with built-in tools and end up shadowing core functionality.
2. tool_search groups namespaces through a map that can reorder namespaces alphabetically instead of preserving highest-match relevance, and its search text currently misses some connector/plugin metadata.

Why this matters

• A connector or plugin can accidentally shadow built-in tools such as update_plan, which changes behavior in a way that is hard to diagnose.
• tool_search can return the right namespaces in the wrong order, which makes discovery feel noisy and less trustworthy.
• Searching by connector id or plugin display name can miss relevant tools even when the tool is available.

Proposed fix

I implemented a candidate fix on my fork with two small reviewable commits:

• a95bb73 fix(core): guard builtin tools from external name collisions
• 332ab81 fix(core): stabilize tool search metadata and ordering

The changes do the following:

• skip registration of external tools / aliases when they would collide with a built-in tool name or handler key
• preserve tool_search namespace ordering by best relevance rather than map key order
• keep tool ordering stable inside a namespace
• index connector_id and plugin_display_names in tool_search
• append plugin provenance to namespace descriptions
• add regression coverage for collision handling and tool search metadata/order

Ready branch

• Fork branch: JaysonGeng:fix/tool-discovery-stability-and-collision-guards
• Compare URL: https://github.com/openai/codex/compare/main...JaysonGeng:codex:fix/tool-discovery-stability-and-collision-guards?expand=1

Validation I ran locally

• cargo test -p codex-core --no-run
• targeted codex-core regression tests covering tool_search, search_tool, and collision handling
• just fix -p codex-core
• just fmt

I also ran cargo test at the codex-rs workspace level. The only remaining failure on my machine was an existing codex-app-server integration test unrelated to this branch's diff:

• suite::v2::command_exec::command_exec_process_ids_are_connection_scoped_and_disconnect_terminates_process

This branch only changes the following files under codex-rs/core:

• codex-rs/core/src/tools/handlers/tool_search.rs
• codex-rs/core/src/tools/handlers/tool_search_metadata_tests.rs
• codex-rs/core/src/tools/registry.rs
• codex-rs/core/src/tools/spec.rs
• codex-rs/core/src/tools/spec_collision_tests.rs

Request

If this direction aligns with the team's roadmap, would you be open to either:

• inviting a PR from the branch above, or
• cherry-picking the two commits directly

I can also rework the patch if there is a preferred design direction.