feat(client): exponential backoff retry for transient 5xx errors

[bennytimz]

Summary

Adds configurable exponential-backoff retry for transient 5xx errors to PaymentTransport and Client. This builds on the 402 retry work introduced in PR #156 and PR #166.

• New RetryPolicy dataclass — configurable max_attempts, backoff_delays, retryable_status_codes, and retry_on_connect_error
• PaymentTransport — accepts retry_policy: RetryPolicy | None = None; a private _dispatch() method runs the retry loop before the 402 payment flow, so transient server errors are resolved before any payment credential is created
• Client — retry_policy defaults to RetryPolicy() (on by default with sensible defaults); pass retry_policy=None for raw no-retry behaviour
• RetryPolicy exported from both mpp and mpp.client for convenient imports

Design notes

Retry is scoped to the initial dispatch only. The 402 paid-retry request is not subject to 5xx retry — it is a distinct operation that has already incurred payment.

Async-generator bodies (guarded by PR #166) raise PaymentError before the dispatch loop, so the retry path never encounters a half-consumed stream.

RetryPolicy is on by default in Client. Users who want zero-overhead raw behaviour pass retry_policy=None.

# Default: retry on 500/502/503/504 up to 3 times with 0.5s, 1.0s backoff
async with Client(methods=[tempo(...)]) as client:
    response = await client.get(url)

# Custom policy
async with Client(methods=[tempo(...)], retry_policy=RetryPolicy(max_attempts=5)) as client:
    ...

# Disable retry
async with Client(methods=[tempo(...)], retry_policy=None) as client:
    ...

Test plan

• test_5xx_retried_with_backoff — 503×2 then 200; asserts asyncio.sleep called with 0.5 and 1.0
• test_5xx_exhausted_returns_last_response — max_attempts=2, always 503; asserts 503 returned after 2 calls
• test_connect_error_retried — ConnectError then 200; asserts final response is 200
• test_retry_disabled_when_policy_is_none — PaymentTransport(retry_policy=None), 503 returned immediately
• test_non_retryable_5xx_not_retried — 501 not in retryable_status_codes; single attempt
• test_client_retry_disabled_when_policy_is_none — Client(retry_policy=None) via HTTPXMock
• All 29 tests in tests/test_client.py pass (24 existing + 5 new retry tests + 1 existing)