CVS-174486: [OV EP] Followup: Add tests for dynamic shapes and for empty initializers

[intbf]

Introduce OVEP_ExtInit_DynamicEmbed_Tests to verify >2GB external weights handling and expected protobuf exception. Add OVEP_ExtInit_EmptyRawData_Tests to check correct backend behavior with empty raw_data initializers.

Relates to: https://jira.devtools.intel.com/browse/CVS-174486

[Copilot]

Pull request overview

This PR adds two new test fixture classes and corresponding test cases to openvino_ep_ext_init.cc to improve coverage of the OpenVINO EP's external initializer handling:

1. OVEP_ExtInit_DynamicEmbed_Tests: Tests that creating a session with a model that has dynamic shapes and >2GB external weights (9 × 256MB = ~2.25GB) throws an Ort::Exception, since the OV EP must embed the weights into the protobuf which then exceeds protobuf's 2GB limit.

2. OVEP_ExtInit_EmptyRawData_Tests: Tests that a model containing empty raw_data initializers (like empty tensors used as Resize inputs) is correctly handled by the OpenVINO EP backend, without crashing or producing incorrect output.

Changes:

• Introduces OVEP_ExtInit_DynamicEmbed_Tests fixture with SetUpTestSuite/TearDownTestSuite and a test verifying that Ort::Exception is thrown for >2GB weight embedding
• Introduces OVEP_ExtInit_EmptyRawData_Tests fixture with SetUpTestSuite/TearDownTestSuite and a test verifying correct inference output when empty raw_data initializers are present
• Registers both new parameterized test suites for CPU, GPU, and NPU device variants

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