feat(edifabric): surface retry_after_seconds on quota-blocked responses

EdifabricError now carries an optional retry_after_seconds kwarg
parsed from the upstream Retry-After header (API Management quota
policy always sends this on 429/403 quota responses). The spot-check
driver threads the value into each transport_error entry of
spot-check.json so the operator sees the exact moment quota
replenishes rather than parsing it out of the response body.

Adds a MockTransport test confirming retry_after_seconds is set
when the upstream sends Retry-After. Existing callers are
unaffected (retry_after_seconds defaults to None and is keyword-only).

backend/tests/test_edifabric.py

@@ -136,6 +136,28 @@ def test_validate_interchange_raises_on_5xx():
     assert "upstream overloaded" in str(exc_info.value.body)
 
 
+def test_read_interchange_raises_with_retry_after_when_quota_blocked():
+    """On HTTP 403 with a ``Retry-After`` header (API Management quota
+    policy), the raised EdifabricError exposes ``retry_after_seconds``
+    so callers can sleep exactly until quota replenishes."""
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(
+            403,
+            headers={"Retry-After": "50492"},
+            json={"statusCode": 403,
+                  "message": "Out of call volume quota."},
+        )
+
+    _install_factory(handler)
+    with pytest.raises(edifabric.EdifabricError) as exc_info:
+        edifabric.read_interchange(b"ISA*...", api_key=_TEST_KEY)
+    err = exc_info.value
+    assert err.status_code == 403
+    assert err.retry_after_seconds == 50492
+    assert "Out of call volume quota" in str(err.body)
+
+
 # --- validate_edi (composed) ------------------------------------------