Extract tool-search output helpers into codex-tools

[bolinfest]

Why

This is the next straight-refactor step in the codex-tools migration that follows #16493.

codex-rs/core still owned a chunk of pure tool-discovery metadata and response shaping even though the corresponding tool_search / tool_suggest specs already live in codex-rs/tools. Per the guidance in AGENTS.md, this moves that crate-agnostic logic out of codex-core so the handler crate keeps only the BM25 ranking/orchestration and runtime glue.

What changed

• Moved the canonical tool_search / tool_suggest tool names and the tool_search default limit into codex-rs/tools/src/tool_discovery.rs.
• Added ToolSearchResultSource and collect_tool_search_output_tools() in codex-tools so namespace grouping and deferred Responses API tool serialization happen outside codex-core.
• Rewired ToolSearchHandler, ToolSuggestHandler, and core/src/tools/spec.rs to consume those exports directly from codex-tools.
• Ported the existing tool_search serializer tests from core/src/tools/handlers/tool_search_tests.rs to tools/src/tool_discovery_tests.rs without adding new behavior coverage.

Validation

cargo test -p codex-tools
cargo test -p codex-core tools::spec::tests
just argument-comment-lint