Fix retry logic for POST requests

[hownowstephen]

Summary

• Enable retries for POST requests: urllib3's Retry defaults exclude POST from allowed_methods, so all SDK retries were silently no-ops. Sets allowed_methods=None to retry all HTTP methods.
• Retry on server errors: Adds status_forcelist=[500, 502, 503, 504] so transient 5xx responses trigger retries.
• Accept all 2xx status codes: Previously only 200 was accepted; 201, 202, 204 etc. raised exceptions.
• Fix misleading error wrapping: 4xx errors from the API were caught by the generic except Exception and wrapped in "Failed after N retries" — now CustomerIOException is re-raised directly.

Context

This is the most-reported issue in the repo (#76), active since 2021 with users hitting ConnectionResetError in production. The root cause was identified by @skion: retry parameters were decorative for POST.

Closes #76

Test plan

• test_retry_config_allows_post — verifies allowed_methods=None and status_forcelist on built session
• test_non_200_raises_without_retry_wrapper — 400 errors raise clean CustomerIOException without "retries" noise
• test_2xx_status_codes_accepted — 200/201/202/204 all pass through
• Full test suite passes (make test — 31 tests)
• Lint passes (make lint)

---

Note

Medium Risk
Touches core HTTP request/retry behavior; incorrect retry semantics or status handling could change how clients react to transient failures or non-200 successes.

Overview
Fixes ClientBase request handling so all 2xx responses are treated as success (not just 200) and CustomerIOException from non-2xx responses is no longer wrapped in a misleading "failed after N retries" message.

Updates the underlying urllib3 Retry configuration to actually retry POST/other methods (allowed_methods=None) and to retry on selected transient server errors via status_forcelist (500/502/503/504), with new unit tests covering retry config, non-2xx exception behavior, and acceptance of 201/202/204.

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