fix: retry a raw retriable asyncpg error, so connect-time failures retry on every sqlalchemy

[lesnik512]

Replaces #57, which GitHub closed when its stacked base (#56) merged.

Problem

_is_retriable_link only recognised a retriable asyncpg error wrapped in a DBAPIError. Errors raised while SQLAlchemy is executing a statement always arrive wrapped. Errors raised by an async_creator at connect time arrive differently depending on the SQLAlchemy version:

Factory raises	SQLAlchemy 2.0.54	SQLAlchemy 2.1.0
asyncpg.PostgresConnectionError (class 08)	raw, not retried	DBAPIError, retried

So the same db-retry release retried a connect-time lost connection on 2.1 and gave up on 2.0. The 2.1 behaviour is the one CONTEXT.md describes ("a lost connection (class 08)" is retriable, and RETRIABLE_ASYNCPG_ERRORS "is the whole of it"). The 2.0 gap came from the wrapper check, not from a deliberate rule.

Change

A chain link is now retriable when the asyncpg error it carries, either the raw exception or a DBAPIError's orig.__cause__, is in RETRIABLE_ASYNCPG_ERRORS. It is still one isinstance against the tuple, as ADR 0002 describes, and the 40003 invariant is unchanged; a raw StatementCompletionUnknownError case pins it.

Behaviour change for SQLAlchemy 2.0 users: under postgres_retry, a connect-time PostgresConnectionError (and subclasses) is now retried. A raw SerializationError or PostgresConnectionError raised anywhere in the chain, including from direct asyncpg use inside a decorated coroutine, is now retried too.

Tests

• test_is_retriable: raw 40001 and 08000 are retriable; raw 40003 and a raw base PostgresError are not.
• test_a_connect_time_connection_error_is_retried: an async_creator that raises PostgresConnectionError is called once per attempt under postgres_retry.

Before the fix, the raw-error cases failed on both versions and the connect-time test failed on 2.0.54 only. After it, the full suite passes against Postgres at 100% coverage on sqlalchemy 2.0.54 and 2.1.0 (versions confirmed from inside the test interpreter); just lint-ci green.