[Feature]: Add benchmark decomposition for REST and JSON-RPC paths

[MisterVVP]

Benchmark signal

Current full-path transport benchmarks show useful timings, but they mix routing, parsing, task lifecycle, protobuf conversion, and response serialization.

Task

Split high-level transport benchmarks into component benchmarks.

Scope

Add or improve benchmarks for:

• JSON-RPC envelope parse only
• JSON-RPC envelope serialize only
• protobuf-to-JSON only for each response shape
• REST query parser only
• REST response builder only
• task-store projection only
• task lifecycle only without serialization

Success criteria

• Each high-level hotspot has at least one lower-level benchmark explaining where time is spent.
• Roadmap optimization tasks can be tracked against component benchmarks, not only end-to-end transport benchmarks.
• CI summary remains readable.