chore: replace always-on faraday-retry with opt-in RetryConfig

[mogita]

Ticket

https://linear.app/stream/issue/CHA-2959/rate-limits-and-retry

Summary

Adds an opt-in auto-retry policy. RetryConfig is disabled by default (maxAttempts=3, maxBackoff=30s). When enabled, retries ONLY GET/HEAD requests that fail with HTTP 429 or a transport error, honoring Retry-After (clamped to maxBackoff) otherwise exponential backoff with full jitter. Never retries writes or 5xx; always honors the backend unrecoverable flag; surfaces the last attempt's error. Config surface: retry_config: GetStreamRuby::RetryConfig.new(...)

Notes

• Behavior change: removes the always-on faraday-retry middleware (and its gemspec dependency); retries are now opt-in only.
• Retry attempts are observable via the existing http.request.failed log event with a retry.attempt field (transport retries carry error.type; 429 retries omit it, since that field is the transport-only enum).
• Titled chore: intentionally so the merge does not auto-publish. Publish later via the manual Release workflow (workflow_dispatch, minor); the release notes should flag the behavior change.