codex-tools: extract MCP schema adapters

[bolinfest]

Why

codex-tools already owns the shared tool input schema model and parser from the first extraction step, but core/src/tools/spec.rs still owned the MCP-specific adapter that normalizes rmcp::model::Tool schemas and wraps structuredContent into the call result output schema.

Keeping that adapter in codex-core means the reusable MCP schema path is still split across crates, and the unit tests for that logic stay anchored in codex-core even though the runtime orchestration does not need to move yet.

This change takes the next small step by moving the reusable MCP schema adapter into codex-tools while leaving ResponsesApiTool assembly in codex-core.

What changed

• added tools/src/mcp_tool.rs and sibling tools/src/mcp_tool_tests.rs
• introduced ParsedMcpTool, parse_mcp_tool(), and mcp_call_tool_result_output_schema() in codex-tools
• updated core/src/tools/spec.rs to consume parsed MCP tool parts from codex-tools
• removed the now-redundant MCP schema unit tests from core/src/tools/spec_tests.rs
• expanded codex-rs/tools/README.md to describe this second migration step

Test plan

• cargo test -p codex-tools
• cargo test -p codex-core --lib tools::spec::