[codex] Handle empty model response choices

[h0rv]

Fixes #66.

Summary

• Guard OpenAI JSON and JSONSchema response handling before reading Choices[0].
• Skip empty OpenAI stream chunks instead of panicking on Choices[0].
• Add matching guards for Anthropic JSONSchema content and schema ref parsing.
• Add a focused OpenAI regression test for empty choices.

Validation

• GOCACHE=/tmp/go-build-cache GOMODCACHE=/tmp/go-mod-cache go test ./...

[h0rv]

@jxnl +1 plz?

[h0rv]

@jxnl would i get be able to get admin access to this repo again please - would make bumping deps + small bug fixes easier as the repo is mostly in maintenance state. Thanks!