test(providers): record live Mailchimp report responses for M6

[TheAggressive]

Part of #67 (M6). These responses were recorded from the live Mailchimp account before its subscription ends, so normalized reporting can be built and tested without the account.

What this adds

• tests/Fixtures/Mailchimp/reports.json records read-only, aggregate endpoints:
  • for a campaign that was sent: the report, click details, domain performance, locations, and sub-reports;
  • the report of an unsent draft (200, with nothing sent);
  • a report for a missing campaign (404 error shape);
  • the audience's stats.
• Redacted before anything was written. Member-level endpoints were not called. Campaign, audience, and web IDs are placeholders, the recipient domain is example.com, list names, subjects, and sender details are {{field}} placeholders, and links are removed.
• Mailchimp_Fixture_Privacy_Test checks every recorded Mailchimp fixture: it must state its source and contain no email address or credential. It also pins the aggregate shapes that reporting will rely on.

Findings for #67

• delivery_status.status says delivered even when the only recipient soft-bounced. It means the send finished, not that mail reached an inbox, so reporting must not show it as inbox delivery.
• Opens come in two forms, raw and proxy_excluded_* (Apple Mail Privacy Protection). Reporting should say which one it shows.
• An unsent draft still returns a 200 report with zero counts. Reporting should not take that as a sent campaign.

Evidence

Mailchimp_Fixture_Privacy_Test passes (4 tests), and Prettier is clean.

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