[codex] Move tool specs onto handlers

[pakrym-oai]

Why

This is the next stacked step after deleting the tool-handler kind indirection. Specs should come from the registered handlers themselves so registry construction has a single source of truth for handler behavior and exposed tool definitions.

What changed

• Added ToolHandler::spec() plus handler-provided parallel/code-mode metadata, and made ToolRegistryBuilder::register_handler automatically collect specs from registered handlers.
• Moved builtin tool spec construction into the corresponding handlers and their adjacent _spec modules, including shell, unified exec, apply patch, view image, request plugin install, tool search, MCP resource, goals, planning, permissions, agent jobs, and multi-agent tools.
• Reworked configurable handlers to receive their tool-building options through constructors, with non-optional handler options where the handler is always spec-backed. Shell fallback handlers keep an explicit no-spec mode because they are also registered as hidden dispatch aliases.
• Kept CodeModeExecuteHandler on the explicit configured wrapper so the code-mode exec spec can still be built from the nested registry.

Verification

• cargo check -p codex-core
• cargo test -p codex-core tools::spec_plan::tests
• cargo test -p codex-core tools::spec::tests
• cargo test -p codex-core tools::handlers::multi_agents_spec::tests
• RUST_MIN_STACK=16777216 cargo test -p codex-core tools::handlers::multi_agents::tests
• cargo test -p codex-core tools::handlers::apply_patch::tests
• cargo test -p codex-core tools::handlers::unified_exec::tests
• just fix -p codex-core
• git diff --check