Support metricsEnhanced dataset for transaction performance metrics

[jds2501-cs]

Problem

The list_events tool currently hardcodes dataset to only ["errors", "logs", "spans"]:
https://github.com/getsentry/sentry-mcp/blob/main/packages/mcp-core/src/tools/list-events/index.ts#L67

However, on self-hosted Sentry instances with metrics-only configuration:

• The spans dataset returns empty (no event-level span storage)
• The Sentry UI uses dataset=metricsEnhanced to display transaction metrics
• The metricsEnhanced dataset works perfectly via direct API calls

Impact

Users cannot access transaction performance data (p50, p75, p95, p99) through MCP, even though:

1. The data exists and is visible in the UI
2. The API endpoint supports it
3. The list_events tool already supports aggregate functions

Proposed Solution

Add "metricsEnhanced" to the dataset enum in list-events/index.ts:

dataset: z
  .enum(["errors", "logs", "spans", "metricsEnhanced"])
  .default("errors")

[dcramer]

I think metrics are moving to a new dataset so I'm not we can reasonably support this in here, but we'll def get the new dataset once its in. Will double check internally tho.