fix(recipe): restore unconditional nlohmann_json; gate cpp_info entry…

[michalmaj]

… on with_onnx

nlohmann_json is used unconditionally by src/ml/coco_dataset.cpp so it must always be in requirements(). However Conan 2's package_info() validation behaves differently across configurations:

• static + with_onnx: onnxruntime also depends on nlohmann_json, making it visible in the graph; Conan requires it listed in cpp_info.requires.
• static/shared + no_onnx: visible=False keeps it quiet; adding it to cpp_info.requires causes "not a direct requirement" on shared builds.

Solution: keep nlohmann_json unconditional in requirements(), but only append "nlohmann_json::nlohmann_json" to cpp_info.requires inside the with_onnx guard.

Summary

Type of change

• feat — new op / feature
• fix — bug fix
• docs — tutorial, example, or doc comment
• bench — benchmark
• chore — version bump, CI, build system, release

Ops / APIs added or changed

Op	Namespace	Header

Test plan

• cmake --build build --parallel — no errors
• cd build && ctest --output-on-failure — all tests pass (or pre-existing failures documented below)
• New ops have GTest coverage in tests/
• New examples build and run without crashing

Pre-existing test failures (if any)

None

CHANGELOG updated?

• Yes — new entry under ## [Unreleased] (or version section for releases)
• N/a — docs / chore only