Expose SourceId on SummaryFieldStandardValue

Summary:
Extend the CE SDK to expose SourceId support on SummaryFieldStandardValue, mirroring the existing behavior on SummaryFieldDefinition. This enables BOE Pro to maintain a stable, order-independent identity for PBS tree nodes submitted as SummaryFieldStandardValue records in CE, even when auto-numbering changes their display names.

Problem Statement:
BOE Pro submits each PBS tree node to CE as a SummaryFieldStandardValue. These entries currently lack a stable identity on the CE side and can only be matched back during sync by their name. When auto-numbering is enabled, PBS node names change whenever users reorder siblings, breaking name-based matching. This leads to unreliable synchronization between BOE Pro and CE and undermines the fixes introduced by US 30515 for auto-numbered fields.

Proposed Solution:
Expose SourceId support on SummaryFieldStandardValue, consistent with the implementation on SummaryFieldDefinition shipped for US 30514:

• Add a public SourceId getter on SummaryFieldStandardValue.

• Add a WithSourceId(Guid value) method that uses the same state guards as SummaryFieldDefinition.WithSourceId (only settable while the entity is new and SourceId is null).

• Ensure the behavior and validation rules align with the existing SummaryFieldDefinition pattern, so clients can rely on a consistent model across both CE entities.

Benefits:

• Provides BOE Pro with a stable, order-independent identity for PBS nodes, eliminating reliance on mutable names for sync.

• Completes the auto-numbering support introduced in US 30515 by ensuring PBS value entries remain matchable after reordering.

• Aligns SummaryFieldStandardValue with the already-shipped SummaryFieldDefinition behavior from US 30514, improving conceptual and API consistency.

• Reduces synchronization errors and maintenance overhead for integrations that depend on CE’s summary field model.