Tracking: one retry primitive for the patch API client (JSON, vendor service, blob and diff) #676

[agent] Filed by the scheduled architecture audit routine (CLI and core). Register: register comment.

Kind: tracking. Source: review Part 7.2 ("Three retry systems in one module") and R12; register row C15.

Problem

ApiClient has two independent retry policies. A third path, the blob and diff downloads, has none.

The same Socket hosts are therefore treated differently depending on the call. Shown by execution on 045d7ec (a temporary unit test, run twice, not committed), with Retry-After: Fri, 27 Mar 2026 19:12:42 GMT:

• api::retry::parse_retry_after waits until that date;
• the vendor path's retry_after_secs returns None and falls back to its own backoff;
• 500, 502 and 504 are retried on vendor calls but are final on JSON calls.

Target design

One retry primitive in api::retry:

• one Retry-After parser and one jitter source;
• a Classifier per call family, since the retryable statuses legitimately differ (vendor archives build asynchronously, the patch API throttles);
• one retry(op, policy, classifier) loop.

VendorRetryPolicy becomes a preset of that policy, not a second implementation, and fetch_binary gets the JSON preset. Timeouts stay where #581 put them (ApiTimeouts).

Checklist (each item one PR, in order)

• Vendor-service retries ignore an HTTP-date Retry-After: fold the vendor Retry-After parser and jitter onto api::retry #677 (child 1): fold the vendor path's Retry-After parser and jitter onto api::retry, deleting retry_after_secs and jitter_sample() from client.rs.
• Child 2: one retry_with loop. Replace the three hand-written vendor loops (request_vendor_package, download_vendor_archive_retrying/vendor_download_retry and download_artifact_capped) and send_json_request's loop, keeping each call family's classifier. Not filed yet; it needs child 1 and Retry patch API connections reset mid-handshake #610.
• Child 3: retry blob and diff downloads (fetch_binary) under the JSON preset. Not filed yet; it is blocked by Stream patch blob and diff downloads to disk (#571) #607 (C37), which rewrites fetch_binary to stream.
• Child 4: merge the two near-identical downloaders, download_vendor_archive_once and download_artifact_capped_once. Not filed yet.

Out of scope: the non-patch-API stacks (RegistryClient, self-update and telemetry clients). Telemetry's per-event client is C22.

Dependencies

• Blocked in part by Stream patch blob and diff downloads to disk (#571) #607 (child 3) and Retry patch API connections reset mid-handshake #610 (child 2: transport retry for JSON calls).
• Related: The public-proxy per-package fallback runs 10 requests in flight, ignoring SOCKET_API_CONCURRENCY and the proxy cap of 4 #614 (C38, pacing) and Only scan, get <uuid> and vex fall back to the public proxy on 401/403; get search, apply, rollback, repair and vendor eject fail #647 (C39, proxy fallback in ApiClient).