Standardize naming conventions and reorganize test directory structure

[timenick]

Summary

• Acronym casing unified across 9 class names (18 source files): OnnxOP→ONNXOp, OP*→Op*, Ep*→EP*, Qdq*→QDQ*, OnnxConfigNotFoundError→ONNXConfigNotFoundError, OnnxModelOutput→ONNXModelOutput
• Test directory reorganized: moved 18 flat module dirs from tests/ root into tests/unit/, merged duplicate onnx/ and sysinfo/ dirs, renamed dataset_tests/ to datasets/
• CI workflow updated: test group paths in modelkit-ci.yml aligned to new structure
• Naming convention documented: added docs/naming-convention.md (referenced from CLAUDE.md) and tests/CLAUDE.md for test directory rules

Details

Naming fixes

Before	After	Acronym
OnnxOP	ONNXOp	ONNX + Op
OnnxConfigNotFoundError	ONNXConfigNotFoundError	ONNX
OnnxModelOutput	ONNXModelOutput	ONNX
EpContextNodeChecker	EPContextNodeChecker	EP
EpPackage	EPPackage	EP
QdqFixResult	QDQFixResult	QDQ
OPOptionalInputSupportError	OpOptionalInputSupportError	Op
OPLackOfRequiredInformationError	OpLackOfRequiredInformationError	Op
OPUnsupportedError	OpUnsupportedError	Op

Test structure (before → after)

tests/                          tests/
├── build/          ──→         ├── unit/
├── cache/          ──→         │   ├── analyze/
├── commands/       ──→         │   ├── build/
├── compiler/       ──→         │   ├── cache/
├── config/         ──→         │   ├── commands/
├── core/           ──→         │   ├── compiler/
├── dataset_tests/  ──→         │   ├── config/
├── eval/           ──→         │   ├── core/
├── export/         ──→         │   ├── datasets/
├── inspect/        ──→         │   ├── eval/
├── loader/         ──→         │   ├── export/
├── models/         ──→         │   ├── inspect/
├── onnx/           ──→         │   ├── loader/
├── optim/          ──→         │   ├── models/
├── optracing/      ──→         │   ├── onnx/
├── session/        ──→         │   ├── optim/
├── sysinfo/        ──→         │   ├── optracing/
├── unit/           ──→         │   ├── session/
│   ├── analyze/                │   ├── sysinfo/
│   ├── onnx/                   │   └── utils/
│   └── sysinfo/                ├── integration/
├── integration/                ├── e2e/
├── e2e/                        ├── regression/
├── regression/                 ├── assets/
├── test_cli.py     ──→        ├── fixtures/
└── test_text_...   ──→        └── mock_data/