Remove hard core ORT dependency from CUDA plugin EP packages

[tianleiwu]

Description

The CUDA plugin EP packages (onnxruntime-ep-cuda12/onnxruntime-ep-cuda13 wheels and the
Microsoft.ML.OnnxRuntime.EP.Cuda NuGet package) currently declare a hard dependency on the core
onnxruntime package. That is wrong for onnxruntime-gpu users, who would otherwise be forced to pull in
the CPU onnxruntime/Microsoft.ML.OnnxRuntime package. This change drops the hard dependency from both
packages, documents the core-package prerequisite in the READMEs, and installs/pins the core ORT package
explicitly in CI — mirroring what was done for the WebGPU plugin EP in #28384.

The minimum compatible ORT version remains the single source of truth in
plugin-ep-cuda/MIN_ONNXRUNTIME_VERSION; it is now injected into
each package's README at build/pack time, and the native plugin EP validates compatibility at registration
time.

Summary of Changes

Python wheel

File	Change
plugin-ep-cuda/python/pyproject.toml.in	Removed the dependencies = ["onnxruntime>=..."] block.
plugin-ep-cuda/python/build_wheel.py	Import the shared gen_file_from_template helper; render the staged package README with the minimum ORT version; stop injecting the version into pyproject.toml.
plugin-ep-cuda/python/onnxruntime_ep_cuda/README.md	Added a Prerequisites section and an explicit pip install "onnxruntime>=@min_onnxruntime_version@" step.

C# NuGet package

File	Change
plugin-ep-cuda/csharp/Microsoft.ML.OnnxRuntime.EP.Cuda/Microsoft.ML.OnnxRuntime.EP.Cuda.csproj	Removed the OnnxRuntimeMinVersion resolution/validation machinery and the hard Microsoft.ML.OnnxRuntime PackageReference.
plugin-ep-cuda/csharp/Microsoft.ML.OnnxRuntime.EP.Cuda/README.md	Added a Prerequisites section and templatized the version requirement.
plugin-ep-cuda/csharp/pack_nuget.py	Render the staged README via the shared helper; dropped the -p:OnnxRuntimeMinVersionFile plumbing from dotnet build/pack.

Shared / docs

File	Change
plugin-ep-cuda/_packaging_utils.py	New shared gen_file_from_template helper (matches the WebGPU plugin's utility).
plugin-ep-cuda/README.md	Documented the no-hard-dependency behavior and the C# package.

Tests & CI

File	Change
plugin-ep-cuda/csharp/test/CudaEpNuGetTest/CudaEpNuGetTest.csproj	Reference the core Microsoft.ML.OnnxRuntime package explicitly via $(OrtCoreTestVersion) (no longer transitively pulled in).
tools/ci_build/github/azure-pipelines/stages/plugin-linux-cuda-test-stage.yml	Dropped the now-unnecessary --no-deps install flag and updated the comment.
tools/ci_build/github/azure-pipelines/stages/plugin-win-cuda-test-stage.yml	Set OrtCoreTestVersion from MIN_ONNXRUNTIME_VERSION and pass it to dotnet build.

Testing

• python -m ruff check and lintrunner pass on the changed files.
• Python scripts compile (python -m py_compile).
• CI: the Linux/Windows CUDA plugin test stages install the core onnxruntime package explicitly and verify
  the plugin wheel installs without altering the pinned core version; the NuGet test project references the
  core package explicitly and pins it to the minimum supported version.

Motivation and Context

Follow-up to PR #28824 (CUDA plugin EP minimum ORT version / version-gated callbacks), kept separate to keep
that PR focused on the version-gating change. Mirrors the WebGPU plugin EP packaging change in #28384. The
CUDA case is stronger: a hard dependency on the CPU onnxruntime package is incorrect for onnxruntime-gpu
users.

Checklist

• Tests added/updated (CI test stages and NuGet test project updated)
• Documentation updated (package and top-level READMEs)
• No breaking changes (consumers must now install the core ORT package explicitly, documented in READMEs)
• CI passes