fix: surface internal API stream errors

[snowyukitty]

Description

Fixes #2176.

When the /inference proxy exhausts its models, it sends an SSE frame with an
error field and no choices. The client previously dereferenced
choices[0] for every parsed frame, replacing the useful server message with a
TypeError.

This change:

• throws ChatGenerationError with the server-provided message for string
  error frames;
• safely skips choice-less and non-content frames; and
• adds focused regression coverage for both behaviors.

The normal content path is unchanged. Chunk-boundary buffering tracked in
#2177 remains outside this patch.

Type of Change

• Bug fix
• New feature
• Documentation
• Other (refactor, build, chore)

Checklist

• npm run lint passes
• Tests pass (npm run test), with tests added where it made sense

Security, performance, or breaking changes? Expand if relevant.

None. The patch changes only client-side handling of an existing server error
frame and adds no new API, dependency, or migration requirement.