feat: add http retries, user-agent, and correlation id

[harley-poly]

summary

hardens the http client (sync + async) for production use:

• automatic retries with exponential backoff + jitter for idempotent requests (GET/DELETE) on connection errors, timeouts, and 408/409/429/5xx. configurable via max_retries (default 2).
• non-idempotent requests (order placement, etc.) are never auto-retried, since the api has no idempotency key and a retry could submit a duplicate order.
• every request now sends a User-Agent (polymarket-us-python/<version>) and a generated poly-correlation-id for server-side tracing.
• errors now carry request_id (from the response or the generated correlation id) alongside the existing status_code/body.

no Retry-After/X-RateLimit-* headers are emitted by the api today, so backoff is purely client-side (the parser honors Retry-After defensively if it ever appears).

test plan

• ruff check / ruff format --check
• mypy polymarket_us
• pytest (126 passed; new tests/test_retries.py covers retry-then-succeed, exhausted retries, connect-error retry, post-not-retried, 4xx-not-retried, request-id, and header propagation; sync + async)

---

Note

Medium Risk
Touches core request paths for trading clients; POST skip-on-retry is intentional but behavior change for timeouts/5xx on reads vs writes.

Overview
Hardens the sync and async HTTP clients for production: automatic retries (configurable max_retries, default 2) on transient failures for idempotent methods only (GET/DELETE and similar), with client-side exponential backoff and jitter; POST (e.g. order placement) is never auto-retried to avoid duplicate submissions without idempotency keys.

Adds shared polymarket_us/_retry.py, sends User-Agent and per-request poly-correlation-id, and surfaces request_id on APIError subclasses (from response headers or the generated correlation id). Connection handling broadens from ConnectError to TransportError.

README documents retries and tracing; new tests/test_retries.py covers retry/backoff/header behavior; HTTP error tests use max_retries=0; auth test fixes 64-byte key fixture encoding.

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