[FSSDK-12012] expose prediction endpoint

[Mat001]

• expose cmab prediction endpoint for server side SDKs so customers can define their own proxy for testing.
• it has been exposed via datafile manager, works similarly to datafile URL template, new URL is added to config, then config is called in optimizely instance

Jira ticket:
https://optimizely-ext.atlassian.net/browse/FSSDK-12012

[Copilot]

Pull Request Overview

This PR adds support for configurable CMAB prediction endpoints, allowing users to specify custom endpoint URLs instead of being hard-coded to the default Optimizely endpoint. This is useful for scenarios like proxying requests through custom infrastructure or using alternative prediction services.

Key changes:

• Refactored the global mutable CMABPredictionEndpoint variable into an instance-level configuration field
• Added PredictionEndpoint field to CMAB configuration structs throughout the codebase
• Updated tests to use the new configuration approach instead of mutating global state

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/cmab/config.go	Added DefaultPredictionEndpointTemplate constant and PredictionEndpoint field to Config struct
pkg/cmab/client.go	Replaced global CMABPredictionEndpoint variable with instance field and DefaultPredictionEndpoint constant; updated ClientOptions to include PredictionEndpoint
pkg/cmab/client_test.go	Refactored tests to use PredictionEndpoint option instead of mutating global state; added new tests for custom endpoint functionality
pkg/decision/experiment_cmab_service.go	Added logic to handle PredictionEndpoint configuration with appropriate fallback to defaults
pkg/client/factory.go	Added PredictionEndpoint field to public CmabConfig struct and its conversion logic
pkg/client/factory_test.go	Added tests for PredictionEndpoint configuration at the client factory level

Comments suppressed due to low confidence (1)

pkg/cmab/config.go:60

• The NewDefaultConfig function does not set the PredictionEndpoint field despite it being part of the Config struct. This means users calling this function won't get the default prediction endpoint. Add PredictionEndpoint: DefaultPredictionEndpoint to the returned Config.

// NewDefaultConfig creates a Config with default values
func NewDefaultConfig() Config {
	return Config{
		CacheSize:   DefaultCacheSize,
		CacheTTL:    DefaultCacheTTL,
		HTTPTimeout: DefaultHTTPTimeout,
		RetryConfig: &RetryConfig{
			MaxRetries: DefaultMaxRetries,
		},
	}
}

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[pvcraven]

Adds CMAB endpoint.