fix(responses): reject incomplete terminal streams

[eclipse1228]

What

Reject terminal response.incomplete and response.failed SSE events from the Responses API instead of reducing them to an empty successful stop response.

Why

A Responses request can return HTTP 200 while ending unsuccessfully, for example when it exhausts max_output_tokens. Previously that stream was treated as a normal empty assistant answer, which could let the agentic loop continue from a false success state.

How

• Capture terminal incomplete and failed events while consuming the SSE stream.
• Preserve the provider diagnostic reason/message when present.
• Throw after the stream is consumed so callers receive a failed run rather than an empty completion.
• Add a regression test for an HTTP 200 stream ending in response.incomplete.

Testing

• npm test -- --run src/adapters/codexResponses.test.ts
• npm run typecheck
• npm run build
• git diff --check

The full suite could not complete locally because the existing better-sqlite3 native binary was built for a different Node ABI; the focused suite and static checks above pass.

Related discussion: #556