feat(events): Add --json output to events dropped

[manessaraj]

Stacked on #3431 (the human table). Adds a --json flag to events dropped for agents and automation, since the terminal table is not machine-readable.

The JSON mirrors the same sectioned structure the table shows: an array of outcomes (outcome code, human label, dropped count, share) each carrying its reason rows (reason, category, dropped count, share). Share is a raw ratio (0.1, not "10%") so agents don't parse formatted strings. Both views are built from one function, so they never diverge.

$ sentry-cli events dropped --org acme --project web --json
[
  {
    "outcome": "client_discard",
    "label": "Client Discard",
    "events": 100,
    "share": 0.1,
    "reasons": [
      {"reason": "sample_rate", "category": "error", "events": 100, "share": 0.1}
    ]
  }
]

Follows the existing per-command --json convention in the CLI (e.g. debug-files check --json); there is no global agentic auto-detection to hook into.

Refs: DAIN-1940

[linear-code]

DAIN-1940

[github-actions]

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