Follow implicit exception chains (__context__) in retry_if_exception_cause_type

[Str0k]

When an exception is raised inside an except block without an explicit raise ... from ..., Python stores the in-flight exception in context and leaves cause as None. retry_if_exception_cause_type only inspected cause, so such implicitly chained exceptions were never retried even when the underlying cause type matched, even though tracebacks render the chain ("During handling of the above exception, another exception occurred"). The predicate now falls back to context when no explicit cause is set and the context was not suppressed with raise ... from None, so both spellings behave identically.

Users writing except SomeError: raise WrapperError(...) (no from) reasonably expect retry_if_exception_cause_type(SomeError) to keep retrying, exactly as the explicit from SomeError form does. The implicit form is the more common spelling in application code, so the predicate silently skipped retries in exactly those cases. Related to issue #625 (retry_unless_exception_cause_type feature request): an inverted predicate would inherit this same gap, so fixing the cause-chain traversal in the existing class grounds both.

Validation: a new regression test fails on the unchanged base and passes with this patch; reproduced in an independent clean checkout.

Full-suite results (including pre-existing failures, if any):

{
  "base": {
    "exit": 1,
    "tests": 203,
    "failures": 1,
    "errors": 0,
    "skipped": 0,
    "failed_cases": [
      "tests.test_retry_if_exception_cause_type_context.test_implicit_context_chain_is_retried"
    ]
  },
  "patch": {
    "exit": 0,
    "tests": 203,
    "failures": 0,
    "errors": 0,
    "skipped": 0,
    "failed_cases": []
  }
}

AI assistance: implementation and review used Hermes with self-hosted GLM 5.3. Automated test evidence was checked separately. This does not represent a human review.

Additional checks were reproduced on fresh base and patched checkouts:

Check	Base	Patch
mypy	0 diagnostics	0 diagnostics
ruff	0 diagnostics	0 diagnostics
ruff_format	exit 0	exit 0
docs_doctest	exit 0	exit 0
docs_html	exit 0	exit 0