fix(alerts): raise ValidationError for deprecated sessions dataset

[billyvg]

Summary

Fixes an unhandled KeyError (SENTRY-5PEF) in the alert rule creation endpoint that surfaces as a 500 error for users attempting to create crash rate alerts with the legacy sessions dataset.

Root cause: validate_dataset in SnubaQueryValidator accepts any valid Dataset enum value — including the deprecated Dataset.Sessions — and passes it along. In _validate_query, line 321 does a dict lookup:

query_type = data.setdefault("query_type", query_datasets_to_type[dataset])

Dataset.Sessions is not in query_datasets_to_type (which maps to alert types), so this raises an unhandled KeyError instead of a user-friendly validation error.

Fix: Add an explicit check for Dataset.Sessions in validate_dataset that raises a clear ValidationError directing users to the metrics dataset for crash rate alerts. This is the correct dataset per the current QUERY_TYPE_VALID_DATASETS mapping (only Dataset.Metrics is valid for CRASH_RATE type).

Evidence from Sentry:

POST /api/0/organizations/{slug}/alert-rules/
aggregate: "percentage(sessions_crashed, sessions)"
dataset: Dataset.Sessions

KeyError: <Dataset.Sessions: 'sessions'>
  File "sentry/snuba/snuba_query_validator.py", line 321
    query_type = data.setdefault("query_type", query_datasets_to_type[dataset])

Session: https://claude.ai/code/session_01J4LcBkJp5FmPYfxmFcq6ZV

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