feat: Add duplicate SAMPLE_ID validation for timeline files (cBioPort…

[hetanshee]

Ref cBioPortal/cbioportal#10917

Description

• Add duplicate SAMPLE_ID validation for timeline files
• Warn when SAMPLE_IDs appear multiple times in SPECIMEN, SAMPLE ACQUISITION, or SEQUENCING events
• Per cBioPortal docs, these event types render as numbered sample discs, duplicates cause conflicting visualization.

[hetanshee]

I took some time to revisit this and wanted to check my understanding.

My understanding is that #129 covers full-record duplication, while this PR targets duplicate SAMPLE_IDs even when rows are not identical.

I included SPECIMEN with SAMPLE ACQUISITION and SEQUENCING because they seemed closely related in the timeline model, but I’d appreciate guidance on whether that’s the right scope here.

I’m also leaning toward making this an error instead of a warning.