Flatten asynchronous retry predicate chains

[Str0k]

Chaining asynchronous predicates with | or & still nests retry_any / retry_all, despite the flattening introduced in #609. Evaluating a chain of 1,501 predicates raises RecursionError before reaching the leaves.

Flatten matching async containers from both sides of each operator. Keep synchronous operands inside the async container so they are evaluated through the existing async wrapper. Mixed operators retain their grouping.

Validation on Python 3.12 / Windows:

• Four regression cases (OR/AND, left/right association) fail before the fix.
• 190 tests and 15 subtests pass after the fix, including mixed sync/async short-circuit order checks.
• Ruff lint/format and mypy pass for the project.
• All 35 documentation examples pass; HTML documentation builds with warnings treated as errors.

Includes a Reno release note. Codex (GPT-6) assisted with implementation and testing.