tools: infer code-mode namespace descriptions from specs

[pakrym-oai]

Why

Code mode already builds the merged nested ToolSpecs that feed the exec prompt. Keeping a separate tool_namespaces map in the planning path duplicated that metadata and left extra wrapper plumbing in spec.rs.

What changed

• derive code-mode namespace descriptions from the merged ToolSpec::Namespace entries before building the code-mode handlers
• extract build_code_mode_handlers(...) so the code-mode-specific planning stays in one place
• remove tool_namespaces from ToolRegistryBuildParams
• delete the now-unused McpToolPlanInputs wrapper and related test helper plumbing

Testing

• cargo test -p codex-core spec_plan