feat: retry transient gateway 5xx via urllib3 Retry (#139)

[martinkersner]

Closes #139

Summary

First of the roadmap issues. The prod edge intermittently returns 502/503/504 under load — tests/conftest.py already documents and retries this, but only for the test suite, so real consumers got a ServerError on the first transient blip.

Mount a urllib3.util.retry.Retry adapter on the shared Session (the single one introduced in #137):

• Retries retry_statuses (default 502, 503, 504) + connection/read failures, GET only (all endpoints are GET).
• backoff_factor between attempts, honors Retry-After.
• raise_on_status=False so an exhausted retry returns the final response and the SDK's own _handle_exception still raises ServerError — error contract unchanged, no leaked urllib3 MaxRetryError.

On by default (max_retries=3). Tunable via max_retries / retry_backoff / retry_statuses; max_retries=0 disables. Rides through **kwargs, so Datamaxi(api_key, max_retries=...) reaches every sub-client.

Design choices (confirmed before implementing)

• urllib3 Retry on the Session over a manual loop — standard, handles connect/read/status + backoff + Retry-After in one place.
• Enabled by default — the flakiness is real and already documented; opt-out via max_retries=0.

Tests / verification

• New tests/test_retry.py (5 tests): asserts the mounted policy config (total, status_forcelist, backoff, respect_retry_after_header, raise_on_status, GET-only), max_retries=0, tunability, shared adapter, and propagation through Datamaxi.
• responses intercepts above urllib3, so it can't drive retries — verified end-to-end against a local server instead: default client 503,503,200 → one transparent success (3 hits); max_retries=0 → ServerError on the first 503.
• pytest -m "not integration": 139 passed, 11 skipped (was 134 + 5 new).
• black + flake8 clean.

Notes

• Mocked lane unaffected: responses bypasses the adapter Retry, so existing 5xx tests stay fast and still assert ServerError.
• conftest's live-lane retry now sits on top of SDK retry (harmless redundancy); simplifying/removing it is a possible follow-up, left out to keep this PR focused.

Known failures

None.