feat: Add validation for duplicate SAMPLE_IDs in timeline files

[VarshiniGunti]

Fixes: #10917

Summary
Adds validation to prevent duplicate SAMPLE_IDs within Sample Acquisition and Sequencing timeline tracks during data import.

Background
Timeline files currently allow multiple entries with the same SAMPLE_ID in a single track. This leads to ambiguous and incorrect rendering in the patient timeline view, where events may appear duplicated or overlapping for the same sample.

Changes

• Added duplicate detection logic to TimelineValidator
• Track SAMPLE_IDs per timeline track during validation
• Emit a clear validation error when a duplicate is detected, including:
• The duplicated SAMPLE_ID
• The affected timeline track
• Validation continues after logging the error, consistent with existing validator behavior

Tests

• Added unit tests covering:
• Detection of duplicate SAMPLE_IDs in timeline files
• Successful validation when all SAMPLE_IDs are unique

Notes

• Python validator only
• No changes to import flow or downstream processing
• No new dependencies introduced