[Copilot] Add test cases from TypeSpec PR #7693

[Copilot]

This PR adds test cases from microsoft/typespec#7693 to the autorest.python repository to maintain consistency between the two codebases.

Changes Made

New Test Files Added

• test_azure_arm_largeheader.py and async version - Tests for large header scenarios
• test_azure_arm_nonresource.py and async version - Tests for non-resource operations
• test_azure_client_generator_core_deserialize_empty_string_as_null.py and async version - Tests for empty string deserialization
• test_azure_encode_duration.py and async version - Tests for Azure-specific duration encoding

Existing Test Files Updated

• CommonProperties tests: Added error handling test cases for predefined and user-defined errors
• Client Initialization tests: Added commented parent-child client test cases and ParentClient import
• Usage tests: Added commented orphan model serialization test cases

Dependencies Updated

• Updated requirements.txt to include new package dependencies:
  • azure-client-generator-core-deserialize-empty-string-as-null
  • azure-encode-duration
  • azure-resource-manager-large-header
  • azure-resource-manager-non-resource

Context

These changes ensure that the autorest.python test suite includes the same test scenarios as the TypeSpec repository, maintaining consistency across both codebases for several spector scenarios including error handling, large headers, non-resource operations, and empty string deserialization.

Fixes #3106.

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