BI-2959: Use External UID Source for Germplasm External UID References

[humsika]

Description

Story: BI-2959

This PR updates the frontend germplasm External UID display logic to match the BI-2959 backend behavior.

External UID is now resolved using the canonical external reference source "External UID" instead of the germplasm Source / seedSource value. This fixes External UID display in:

• germplasm import preview
• germplasm list / all germplasm views

This PR also adds frontend unit tests for the updated External UID lookup behavior.

Dependencies

bi-web: feature/BI-2959
bi-api: feature/BI-2959

Testing

Verified manually:

• germplasm import preview displays External UID when both Source and External UID are present
• germplasm import preview displays External UID when Source is blank and External UID is present
• germplasm import preview does not display External UID when no canonical External UID reference exists
• committed germplasm UI resolves External UID from the canonical "External UID" reference source

Verified with unit tests:

• externaluid.spec.ts
• germplasmutils.spec.ts

Checklist:

• I have performed a self-review of my own code
• I have tested my code and ensured it meets the acceptance criteria of the story
• I have create/modified unit and/or integration tests to cover this change or tests are not applicable
• I have commented my code, particularly in hard-to-understand areas
• I have either updated the source of truth or arranged for update with product owner if needed https://breedinginsight.atlassian.net/wiki/spaces/BI/pages/1559953409/Source+of+Truth

[HMS17]

Proper External UIDs display locally and code looks good!