feat: display all branches including ambiguous ones and filter snapshots by dataset (#657)

Description

• Remove deduplication logic from GET /branches to return all branches, even with duplicate names across datasets
• Add optional dataset query parameter to GET /snapshots
• Added dataset field to the Branch in UI

Related issue

#657 (closed)

Examples

Snapshot filters:

• GET /snapshots?branch=main
• GET /snapshots?dataset=dblab_pool%2Fdataset_2
• GET /snapshots?branch=main&dataset=dblab_pool%2Fdataset_2

GET /branches

[
  {
    "name": "main",
    "baseDataset": "dblab_pool/dataset_1",
    "parent": "-",
    "dataStateAt": "20251007141651",
    "snapshotID": "dblab_pool/dataset_1@snapshot_20251007141651",
    "dataset": "dblab_pool/dataset_1",
    "numSnapshots": 1
  }
]

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Checklist

• MR description has been reviewed
• MR changes are functionally tested
• MR does NOT have API/CLI changes OR there are API/CLI changes and they have been reviewed & DOCS ARE ADJUSTED (reference doc, etc)
• MR does NOT have UI changes OR there are UI changes and they have been reviewed & UX IS REVIEWED

Closes #657 (closed)