fix: cap Retry-After at 60 seconds and trim trailing slashes from baseURL

[TheGreatAxios]

Summary

• The default extractRetryAfterMs clamps both seconds and HTTP-date Retry-After to [0, 60s], so retry-after: 86400 waits one minute, not a day.
• Trailing slashes on baseURL are trimmed, so http://host/v1/ posts to /v1/embeddings, not /v1//embeddings.
• README notes both.
• Stacked on fix: retry per-attempt timeouts and validate vector dimensions #18.

Verification

• bun run check, bun run build: pass
• bun run test:e2e with EMBEDDING_E2E_BASE_URL=http://100.113.184.123:11434/v1/: 14 pass, 0 fail (live included)
• New tests (86400s Retry-After waits 60s; baseURL ending in // hits /v1/embeddings) failed before the fix
• Node pack smoke: pass

Closes CL-9446