refactor(ecstore): extract the embedded S3 client into rustfs-s3-client (#6627)

The storage engine embedded a ~8.4K-line hand-written S3 HTTP client under crates/ecstore/src/client (rustfs/backlog#1842). That client is a legitimate engine capability — it consumes remote S3-compatible endpoints for ILM tier warm backends and transition targets — but it was misfiled inside the engine, dragging s3s/hyper wire types into ecstore and blocking ARCHITECTURE.md invariant 4.

This PR is the pure-move step: 21 modules move verbatim to the new crates/s3-client crate (rustfs-s3-client), and crates/ecstore/src/client/mod.rs becomes a re-export shim so every in-crate crate::client:: path keeps working. The two server-side modules that were historically misfiled under client/ — object_api_utils.rs and object_handlers_common.rs — stay in ecstore.

Three reverse dependencies from the client into engine internals are severed so the move can be pure:

- transition_api::ReaderImpl::ObjectBody held ecstore's GetObjectReader; the client only ever reads the body, so the variant now holds an ObjectReader newtype over Box<dyn AsyncRead + Send + Sync + Unpin> with the same read_all() surface. The single production construction site (set_disk transition upload) and the two engine-side consumers were adjusted.
- api_list/api_remove used ecstore's storage_api_contracts / object_api types; api_list now imports BucketInfo from rustfs-storage-api directly, and api_remove uses the client's own transition_api::ObjectInfo (only .name/.version_id were read; the error-path bucket name is now threaded as a parameter instead of read from the deleted objects).
- the api_put_object_streaming regression tests built a GetObjectReader by hand; they now wrap the duplex stream in ObjectReader::new.

Guard updates: the s3s footprint ratchet gains an ecstore-scoped counter (42 files, shrink-only, per rustfs/backlog#1842), the ecstore module-lint-blanket register follows the moved files into crates/s3-client so the blanket ratchet keeps covering them, the logging guardrail path pin follows transition_api.rs, and the ::other(format!) baseline is regenerated (moved call sites left ecstore).

Verification: cargo check -p rustfs-s3-client -p rustfs-ecstore; cargo nextest run -p rustfs-s3-client (43 passed) and -p rustfs-ecstore (4515/4523; the 8 failures reproduce identically on pristine origin/main on the same machine); cargo clippy --all-targets; scripts/check_layer_dependencies.sh, check_architecture_migration_rules.sh, check_s3s_footprint.sh, check_logging_guardrails.sh, check_error_other_format_ratchet.sh, check_doc_paths.sh, check_ci_paths_sync.sh all pass.

scripts/check_s3s_footprint.sh

@@ -27,6 +27,12 @@ cd "$(dirname "$0")/.."
 # to verify S3 behavior and does not widen the production s3s surface.
 S3S_IMPORT_FILES_BASELINE=211
 S3_ERROR_LINES_BASELINE=1620
+# ecstore-scoped ratchet (rustfs/backlog#1842): the storage engine must not
+# know S3 wire/DTO types (ARCHITECTURE.md invariant 4). The S3-*consuming*
+# client was extracted to crates/s3-client, where s3s usage is legitimate;
+# this counter ratchets the remaining serving-side s3s references out of
+# crates/ecstore. Baseline verified on 2026-08-26.
+S3S_ECSTORE_FILES_BASELINE=42
 S3S_PATH_PATTERN='(^|[^"[:alnum:]_])s3s::'
 E2E_TEST_GLOB='--glob=!crates/e2e_test/**'
 
@@ -47,11 +53,13 @@ run_rg_to() {
 
 run_rg_to "$TMP_DIR/import_files" -l "$S3S_PATH_PATTERN" --type rust $E2E_TEST_GLOB
 run_rg_to "$TMP_DIR/error_lines" -c 's3_error!' --type rust $E2E_TEST_GLOB
+run_rg_to "$TMP_DIR/ecstore_files" -l "$S3S_PATH_PATTERN" --type rust crates/ecstore/src
 
 s3s_import_files="$(grep -c . "$TMP_DIR/import_files" || true)"
 s3_error_lines="$(awk -F: '{sum += $NF} END {print sum + 0}' "$TMP_DIR/error_lines")"
+s3s_ecstore_files="$(grep -c . "$TMP_DIR/ecstore_files" || true)"
 
-for value in "$s3s_import_files" "$s3_error_lines"; do
+for value in "$s3s_import_files" "$s3_error_lines" "$s3s_ecstore_files"; do
     if ! [[ "$value" =~ ^[0-9]+$ ]]; then
         echo "error: could not compute s3s footprint counts (got: '$value')" >&2
         exit 1
@@ -82,6 +90,8 @@ check_ratchet "files importing s3s" "$s3s_import_files" "$S3S_IMPORT_FILES_BASEL
     "rg -l '$S3S_PATH_PATTERN' --type rust $E2E_TEST_GLOB"
 check_ratchet "s3_error! invocation lines" "$s3_error_lines" "$S3_ERROR_LINES_BASELINE" \
     "rg -c 's3_error!' --type rust $E2E_TEST_GLOB"
+check_ratchet "ecstore files referencing s3s" "$s3s_ecstore_files" "$S3S_ECSTORE_FILES_BASELINE" \
+    "rg -l '$S3S_PATH_PATTERN' --type rust crates/ecstore/src"
 
 if ((status != 0)); then
     exit 1