Add RetryableHTTPError and raise_for_retryable_status() to retry.py, then
wire up @retry_with_backoff across all network-touching functions:
- All 5 public Adobe Sign API functions in adobe_api.py
- upload_template() and find_existing_template() in upload_docusign_template.py
raise_for_retryable_status() distinguishes transient errors (429, 500, 502,
503, 504) from auth/client errors — only transient errors are retried.
Auth refresh functions are intentionally left undecorated since a 401 there
means bad credentials, not a transient failure.
Backoff: 1s → 2s → 4s, max 16s, max 3 retries (131 tests passing).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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