Add bt datasets snapshots for dataset version control

[max-braintrust]

TL;DR

Follow up to: #104

Adds bt datasets snapshots for managing saved dataset snapshots from the CLI: list, create, and restore dataset states from saved snapshots.

What changed?

Added dataset snapshot management functionality:

• New bt datasets snapshots command with subcommands: list, create, and restore
• Uses snapshot/dataset snapshot terminology to match the UI
• Snapshot listing displays saved snapshots for a dataset
• Snapshot creation can infer the current dataset head xact when --xact-id is omitted
• Snapshot restore supports restore by snapshot name or by xact ID with --snapshot
• Restore includes preview support and requires confirmation unless --force is passed
• JSON output includes snapshot list/create/restore modes and found_existing for create responses
• Moved snapshot-specific CLI args into the snapshot module to match existing subcommand patterns
• Added shared profile utilities for resolving profile info and generating default snapshot name slugs

Added test coverage:

• Unit tests for snapshot arg parsing, read/write auth routing, response deserialization, default snapshot names, and restore target selection
• Fixture coverage for snapshot creation, duplicate xact handling via x-bt-found-existing, and snapshot listing
• Mock server support for dataset snapshot create and list endpoints

How to test?

# Create a dataset
bt datasets create my-dataset --rows '[{"id":"case-1","input":{"text":"hi"},"expected":"hello"}]'

# List snapshots
bt datasets snapshots list my-dataset

# Create a snapshot
bt datasets snapshots create my-dataset baseline

# Re-run create for the same xact to verify found_existing handling
bt datasets snapshots create my-dataset baseline-again --xact-id 1000192656880881099 --json

# Restore by snapshot name
bt datasets snapshots restore my-dataset --name baseline

# Restore by xact ID
bt datasets snapshots restore my-dataset --snapshot 1000192656880881099 --force

[github-actions]

Latest downloadable build artifacts for this PR commit 56d5516009d7:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/26544295482
• Download all artifacts (GitHub CLI): gh run download 26544295482 --repo braintrustdata/bt
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