fix: eliminate unread clone branches that retain native body buffers (#382)

The 529 in-place retry path cloned the transformed Request on every proxied
request but only read the clone on retry, leaving the tee branch's native
buffer retained per request. Rebuild the retry Request from the already
buffered body text instead. Same pattern fixed in vertex-ai (clone after the
JSON gate), zai (skip cloning non-JSON bodies), and codex (drain, not just
releaseLock, when the SSE transform loop throws). drainReader loosened to a
generic reader type for the decoder-stream case.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>