[FSSDK-11150] cmab client implementation

[junaed-optimizely]

Summary

• Added ICmabClient interface defining the contract for fetching decisions from the CMAB prediction service.
• Implemented DefaultCmabClient class, which interacts with the CMAB service using HttpClient, handles request/response, supports retries, and validates responses.
• Introduced supporting classes: CmabModels for request/response structure, CmabConstants for endpoint and error messages, and CmabRetryConfig for exponential backoff configuration.
• Added new exception classes: CmabException, CmabFetchException, and CmabInvalidResponseException to represent various CMAB client error states.

Test plan

• Added DefaultCmabClientTest unit tests covering success, error, invalid response, and retry scenarios for the CMAB client.
• Updated project files to include new CMAB client, models, constants, and test files for compilation.

Issues

• FSSDK-11150

[Copilot]

Pull Request Overview

This PR implements a CMAB (Contextual Multi-Armed Bandit) client for the Optimizely SDK, providing functionality to fetch decisions from the CMAB prediction service with retry mechanisms and proper error handling.

Key Changes:

• Introduced ICmabClient interface and DefaultCmabClient implementation for interacting with CMAB prediction service
• Added supporting classes for configuration, models, constants, and exception handling
• Implemented comprehensive unit tests for the CMAB client functionality

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
OptimizelySDK/OptimizelySDK.csproj	Added compilation entries for new CMAB client files
OptimizelySDK/Exceptions/OptimizelyException.cs	Introduced CMAB-specific exception classes
OptimizelySDK/Cmab/ICmabClient.cs	Defined interface contract for CMAB client
OptimizelySDK/Cmab/DefaultCmabClient.cs	Implemented HTTP-based CMAB client with retry logic
OptimizelySDK/Cmab/CmabRetryConfig.cs	Configured exponential backoff retry mechanism
OptimizelySDK/Cmab/CmabModels.cs	Defined request/response data structures
OptimizelySDK/Cmab/CmabConstants.cs	Added service endpoints and error messages
OptimizelySDK.Tests/OptimizelySDK.Tests.csproj	Added test file compilation entry
OptimizelySDK.Tests/CmabTests/DefaultCmabClientTest.cs	Comprehensive unit tests for CMAB client
OptimizelySDK.NetStandard20/OptimizelySDK.NetStandard20.csproj	Added CMAB files for .NET Standard 2.0 compilation

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[muzahidul-opti]

Changes look good to me.