Add brand_id support to IDV session configuration

[Copilot]

Adds optional brand_id property to SDK configuration for applying consistent themes across IDV sessions.

Changes

• Added brand_id optional string parameter to SdkConfig class
• Added with_brand_id() builder method to SdkConfigBuilder
• Included brand_id in JSON serialization (auto-removed when null)
• Added unit tests for default behavior, setter, and serialization

Usage

sdk_config = (
    SdkConfigBuilder()
    .with_allows_camera_and_upload()
    .with_brand_id("brand-123")  # Apply branded theme
    .with_success_url("https://example.com/success")
    .with_error_url("https://example.com/error")
    .build()
)

The property follows the same optional parameter pattern as allow_handoff and privacy_policy_url.

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