feat(transport): add HTTP retry with exponential backoff

[jpnurmi]

Add HTTP retry with exponential backoff for network failures, modeled after Crashpad's upload retry behavior.

Note

Adds an explicit 15s connect timeout to both curl and WinHTTP, matching the value used by Crashpad. This reduces the time the transport worker is blocked on unreachable hosts, previously ~75s on curl (OS TCP SYN retry limit) and 60s on WinHTTP.

Failed envelopes are stored as <db>/cache/<ts>-<n>-<uuid>.envelope and retried on startup after a 100ms throttle, and then with exponential backoff (15min, 30min, 1h, 2h, 4h, 8h). When retries are exhausted, and offline caching is enabled, envelopes are stored as <db>/cache/<uuid>.envelope instead of being discarded.

flowchart TD
    startup --> sretry{retry?}
    sretry -->|yes| throttle
    throttle -. 100ms .-> send
    send -->|success| discard
    send -->|fail| retry{retry?}
    retry -->|no| cache{cache?}
    cache -->|no| discard
    cache -->|yes| cache-dump[&lt;db&gt;/cache/<br/>&lt;uuid&gt;.envelope]
    retry -->|yes| cache-retry[&lt;db&gt;/cache/<br/>&lt;ts&gt;-&lt;n&gt;-&lt;uuid&gt;.envelope]
    cache-retry --> backoff
    backoff -. 2ⁿ×15min .-> send

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Builds upon:

• feat(sync): add delayed task submission #1506
• ref(transport): unify and restructure HTTP transport layer #1514

See also:

• Support Offline Caching of Envelopes #1316
• https://develop.sentry.dev/sdk/expected-features/#buffer-to-disk