Add UUID generation and storage to Vulnerabilities::Finding with branch context

Problem

As Vulnerabilities::Finding transitions to represent context-specific instances of vulnerabilities (unique per branch/tag), we need to generate and store UUIDs that include both location fingerprint and security_project_tracked_context_id. This enables easy assertion of branch uniqueness and proper deduplication of findings across contexts.

Background

The vulnerability_occurrences table already has a new_uuid column (added in migration 20251002123802) that will store the context-aware UUID. The existing uuid column will continue to be used until the transition is complete.

UUID composition for Findings (context-aware):

• Report type
• Primary identifier fingerprint
• Location fingerprint
• Security_project_tracked_context_id (branch/tag context)
• Project ID

This ensures each finding is uniquely identified by both its location AND its context, enabling proper deduplication and uniqueness assertions across branches.

Implementation

This issue covers Phase 1: Start populating new_uuid

1. Update Security::VulnerabilityUUID.generate to accept optional security_project_tracked_context_id parameter
2. Add a new method (e.g., generate_with_context) or mode for context-aware UUID generation
3. Modify vulnerability ingestion process to calculate and save finding UUIDs with context into new_uuid column
4. Add comprehensive tests for UUID generation with context

Migration Phases

Phase	Issue	Description
1	This issue	Start populating new_uuid for new findings
2	#587798	Backfill new_uuid for existing findings
3	#587799	Add NOT NULL constraint to new_uuid
4	#587800	Transition GitLab to use new_uuid
5	#587801	Drop old uuid column and rename new_uuid

Acceptance Criteria

• Security::VulnerabilityUUID.generate supports context-aware UUID generation
• New findings ingested with tracked context have new_uuid populated
• Findings for the same vulnerability in different branches have different new_uuid values
• Findings for the same vulnerability in the same branch have the same new_uuid value
• Existing uuid column behavior is unchanged (backward compatible)

Related

• Related to #587624 (Vulnerability UUID - definition model)