Fix POST retries and 504 retry classification

[crmne]

This follows up on #523 with a minimal patch set:

• enable retries for POST while preserving Faraday default idempotent methods
• map HTTP 504 to RubyLLM::ServiceUnavailableError (so retry classification is consistent)
• reduce retry backoff settings in test config to avoid long sleeps in retry/error specs
• add focused specs for retry config and 502/503/504 error mapping

Related: #417