tools: remove is_mutating dispatch gating

[pakrym-oai]

Why

Tool dispatch had two serialization mechanisms:

• supports_parallel_tool_calls decides whether a tool participates in the shared parallel-execution lock.
• is_mutating separately gated some calls inside dispatch.

That second hook no longer carried its weight. The remaining parallel-support flag is already the per-tool concurrency policy, so keeping a second mutating gate made dispatch harder to follow and left behind extra session plumbing that only existed for that path.

What changed

• Removed is_mutating from tool handlers and deleted the tool_call_gate path that existed only to support it.
• Simplified dispatch and routing to rely on the existing per-tool supports_parallel_tool_calls boolean.
• Dropped the now-unused handler overrides and related session/test scaffolding.
• Kept the router/parallel tests focused on the surviving per-tool behavior.
• Removed the unused codex-utils-readiness dependency from codex-core as a follow-up fix for cargo shear.

Testing

• cargo test -p codex-core parallel_support_does_not_match_namespaced_local_tool_names
• cargo test -p codex-core mcp_parallel_support_uses_handler_data
• cargo test -p codex-core tools_without_handlers_do_not_support_parallel

[jif-oai]

lfg