Move the vendoring-service client out of api/client.rs into its own submodule

[mikolalysenko]

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

Kind: refactor (a mechanical move). Source: review Part 7.2 and 7.6 #8, register row C29. This is the first of two moves; the second moves credential resolution out (see Dependencies).

Problem (main @ 0d302dc)

api/client.rs is 6,033 lines: 2,918 production lines, then five test modules. It holds three separate clients behind one impl ApiClient:

1. The patch API JSON client: send_json_request, search, batch, fetch_patch, and blob/diff fetch_binary (L496–L1183).
2. The vendoring-service client, about 895 production lines in three separate places:
   • fetch_vendor_package through download_artifact_resuming and DeferredAttempt: L1185–L1830;
   • its types (MAX_VENDOR_PACKAGE_BYTES, FetchedVendorPackage, SecondaryArtifact, PlannedDownload, PrefetchedSecondary, VendorServiceOutcome, ServeDownload, artifact_download_result): L1834–L1970;
   • a second impl ApiClient after the PatchApi seam (download_artifact_capped, download_artifact_capped_once, CappedAttempt): L2790–L2906;
   • plus its retry policy (VendorRetryPolicy, VENDOR_BREAKER_THRESHOLD, MAX_REFERENCE_BATCH, PYPI_NOT_A_WHEEL, wheel_filename_from_url): L282–L391.
3. Credential and env resolution: ApiClientEnvOverrides, get_api_client_*, resolve_*credentials*, token-shape checks and build_proxy_fallback_client (L2036–L2364).

The vendor tests are just as large: vendor_package_tests has 766 lines and vendor_retry_tests has 762.

Because the vendor client is split across three places in the file, its retry loop (VendorRetryPolicy, three retry_after_secs call sites) is hard to review next to the JSON loop. That is the work #676 and #677 need to do.

Symptoms

None filed. Impact: maintainability. Every change to the patch API client and every change to the vendor service lands in the same file. The C15 retry work (#676, #677) and the C9/C39 proxy fallback work (#647) all edit this file.

Proposed change

• Create api/client/vendor_service.rs as a child module of client, so the moved code keeps access to ApiClient's private fields and helpers without any visibility changes. This means turning client.rs into client/mod.rs, or adding #[path].
• Move all the vendor-service items listed above, with their own impl ApiClient block, into it.
• Move vendor_package_tests and vendor_retry_tests into a sibling api/client/vendor_service_tests.rs, included with #[cfg(test)] #[path] mod.
• Re-export the public names from client so no import outside api/ changes.
• Deleted from client.rs: the second impl ApiClient block after the PatchApi seam, and every vendor-only item. client.rs keeps the JSON client, fetch_binary, the error type and the PatchApi seam.

There are no behavior changes, renames or signature changes. retry_after_secs and jitter_sample() move as they are; #677 deletes them.

Size and scope

• About 895 production lines and about 1,530 test lines move. The net production diff should be about 0 apart from mod/use lines.
• Out of scope:
  • the credential-resolution move (child 2 of C29);
  • the debug-ordering helpers (HeldBack, with_deferred_debug, L95–L180);
  • any change to the retry logic (Tracking: one retry primitive for the patch API client (JSON, vendor service, blob and diff) #676, Vendor-service retries ignore an HTTP-date Retry-After: fold the vendor Retry-After parser and jitter onto api::retry #677).

Acceptance criteria

• grep -c "fn fetch_vendor_package\|fn download_artifact_capped\|struct VendorRetryPolicy" crates/socket-patch-core/src/api/client.rs (or client/mod.rs) prints 0.
• client.rs/client/mod.rs has one production impl ApiClient block for the JSON client, plus the PatchApi impl.
• git diff -M --stat shows the moved tests as moves, and no test bodies changed.
• cargo test -p socket-patch-core --lib api:: passes, as do api_retry_e2e, api_timeout_e2e, binary_fetch_error_classification_e2e and the vendor prefetch tests, with the same test count before and after.
• cargo clippy --workspace --all-features -- -D warnings is clean.

Dependencies

• Land after #865, the only open PR that edits client.rs (13 lines of helpers, outside the moved region), or rebase onto it.
• Vendor-service retries ignore an HTTP-date Retry-After: fold the vendor Retry-After parser and jitter onto api::retry #677 and children 2–3 of Tracking: one retry primitive for the patch API client (JSON, vendor service, blob and diff) #676 become smaller reviews once this lands. They can land before it, but then this move has to absorb their changes.

[mikolalysenko]

[agent] Triaged as priority:p3 (cross-cutting refactor). Confirmed on main 0d302dc: api/client.rs is 6,033 lines with the vendoring-service client still inline. Not a duplicate; no open PR does this move. Sequenced after #865 as the issue says.

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