Align generate-model-package CLI with onnxruntime-genai (microsoft#2495)

This pull request updates the test suite for the model packaging CLI to
reflect a new package layout and metadata schema. The changes remove the
generation and validation of `variant.json` files, update how execution
provider (EP) compatibility and inference settings are represented, and
ensure external data blobs are handled per-variant rather than being
deduplicated. The tests now check for the presence of new fields and
files, and for the absence of deprecated ones.

**Test updates for new package schema and layout:**

* Removed all references to `variant.json`; tests now assert that this
file is not emitted, and instead check for the presence of `model.onnx`
and, where applicable, `genai_config_overlay.json` in variant
directories.
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* Updated assertions for metadata structure:
- EP compatibility is now represented inline within the `variants`
mapping, not as an `ep_compatibility` array.
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- Added checks for new manifest and metadata fields, such as
`schema_version`, `component_name`, `package_name`, `package_version`,
and `configs_dir`.
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**Inference settings and overlay configuration:**

* Runtime and provider options are now placed in
`genai_config_overlay.json` instead of `variant.json`, and tests verify
the correct structure and location of these overlays.
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* Added and updated tests to ensure overlays are omitted for variants
without options, and that provider options are matched to the correct EP
by name.
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**External data handling:**

* Shared weights deduplication has been removed; tests confirm that each
variant keeps its own external data blob inline and that no
`shared_weights` directory or `variant.json` is created.
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**Naming and documentation improvements:**

* Test and comment names have been updated for clarity, reflecting the
new schema and file layout.
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These changes ensure the test suite accurately validates the new
packaging format and metadata conventions.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: xiaoyu-work