[codex] Remove tool search bucket limit override

[sayan-oai]

Why

tool_search still carries the server-specific result-cap path added in #17684 for computer-use: when the model omitted limit, a matching result expanded the search to 20 and then limit_results_by_bucket applied per-bucket caps. That makes default result handling depend on a one-off server exception instead of the single TOOL_SEARCH_DEFAULT_LIMIT path.

This PR removes that custom branch so omitted limit values use the ordinary global default consistently. The implementation being retired is the pre-change bucketed search path in tool_search.rs.

What changed

• Collapse ToolSearchHandler::search back to one BM25 search with the resolved limit.
• Remove limit_results_by_bucket, the computer-use constants, and the omitted-limit plumbing that only existed for the override.
• Drop dead ToolSearchEntry::limit_bucket metadata from deferred MCP and dynamic search entries.
• Remove tests and helpers that only asserted the deleted override behavior.
• Add direct handler-level unit coverage for omitted/default and explicit tool_search result limits.

Validation

• cargo test -p codex-core tool_search
  • The matching unit tests passed, including the new omitted/default and explicit result-limit coverage.
  • The broader --test all search-tool fixture phase then failed before sending mocked response requests in tool_search_indexes_only_enabled_non_app_mcp_tools and tool_search_uses_non_app_mcp_server_instructions_as_namespace_description.
• cargo test -p codex-core
  • The touched tool-search coverage passed before the run later aborted in tools::handlers::multi_agents::tests::tool_handlers_cascade_close_and_resume_and_keep_explicitly_closed_subtrees_closed with a stack overflow.