* test(e2e): cover legacy text /completions endpoint
The /completions (and /v1/completions) text-completion route had zero e2e
coverage despite being the second-busiest endpoint in production; everything
'completions' in the suite was chat. Add a text-completion endpoint test that
registers an OpenAI instruct deployment, drives /v1/completions through the
gateway, and asserts real generated text. Adds text_completions() + the
completion request/result models to EndpointsClient, the 'completions' endpoint
to the coverage registry vocab, and the registry cell.
* test(e2e): assert /v1/completions choices shape, not just joined text
Assert the response carries a choices array and the first choice has real text,
so a malformed response (no choices) and a clean-but-empty completion are
distinct failures. Drop the unused text property / id / model fields (model only
what the test reads).
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