[cx-happy-yang]

check_response() was called inside call_api() before the @Retry decorator could inspect the response, so the 500/502/503/504 and 429 retry branches were dead code — every server error raised ValueError on the first attempt with zero retries (verified against customer logs on 1.9.1: 504 gateway timeout on SCA report export).

Move check_response() into the retry wrapper, after the retry decision:

• 500/502/503/504: honor Retry-After header when present, otherwise exponential backoff (5s, 10s, 20s, ... capped at 30s), retry up to max_retries, then raise the usual ValueError format on exhaustion.
• 429: same treatment — Retry-After first, then exponential backoff (60s base, capped at 300s).
• Non-retryable statuses (e.g. 404) still raise immediately, and success responses pass through unchanged.

Adds tests covering 504 retry-with-backoff, growing backoff, Retry-After, exhausted-retries raising, 429 Retry-After, and 4xx non-retry.

Problem

Customers on 1.9.1 still get 504 gateway timeouts (SCA report export) with zero retries. Traceback verified against tag b2e0095:

api_client.py:271 in call_api -> check_response(response)
api_client.py:204 in check_response -> raise ValueError
ValueError: ('HttpStatusCode: 504', 'ErrorMessage: {"message": "Endpoint request timed out"}')

Root cause: check_response() was called inside call_api() before the @retry decorator could inspect the response, so the 500/502/503/504 and 429 retry branches in the decorator were dead code — every server error raised on the first attempt.

Fix

Move check_response() into the retry wrapper, after the retry decision:

• 500/502/503/504: honor Retry-After header when present, otherwise exponential backoff (5s, 10s, 20s, ... capped at 30s), retry up to max_retries, then raise the usual ValueError format on exhaustion.
• 429: same treatment — Retry-After first, otherwise exponential backoff (60s base, capped at 300s).
• Non-retryable statuses (e.g. 404) still raise immediately; success responses pass through unchanged.

Tests

Six new tests in TestServerErrorRetry covering: 504 retry-with-backoff then success, growing backoff (5s then 10s), Retry-After honored, exhausted retries raising the expected ValueError, 429 with Retry-After, and 4xx raising without retry. tests/test_api_client.py: 13/13 pass.