fix(provider): reject malformed chat completions

[alexsmolya]

Summary

• Reject successful responses without a non-empty choices array, an object first choice/message, and string message.content.
• Guard successful JSON parsing with HTTP status context and a bounded body excerpt for malformed responses.
• Preserve valid responses, non-2xx errors, and network failure handling.

Validation

• Focused provider tests: 13/13 passed.
• Format, lint, typecheck, build, and git diff --check: passed.
• npm run verify: format/lint/typecheck passed; 231 tests passed and one unrelated pre-existing flaky memory property test failed (RangeError: Invalid time value in tests/memory/in-memory-memory-store.property.test.ts). The same failure was reproduced against untouched main; the complete suite is not represented as green.

Fixes #267