Make diagnostic report uploads resilient to slow networks

[copyberry]

Make diagnostic report uploads resilient to slow networks

Why

Diagnostic reports can span several envelopes, and the previous 10-second shared
network budget could expire before slow uploads and their attachments completed.

What changed

• Give each diagnostic report a single five-minute deadline shared by the event,
  attachments, retries, and retry backoff.
• Stop reading or sending later attachments once the deadline or a Sentry rate
  limit is reached.
• Limit the app server to three concurrent report uploads and return an
  overloaded JSON-RPC error for additional requests until a slot is released.

Testing

Add coverage for slow multi-envelope reports, deadline-aware retries, skipped
attachments, rate-limit handling, and concurrency-slot release after failures.

[github-actions]

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