fix(ci): accept DeepSeek reasoning-only reviews

[Sun-sunshine06]

Summary

• accept reasoning_content as a structured-output fallback when DeepSeek returns an empty standard content field
• extract the last complete JSON object from reasoning prose without publishing the reasoning text itself
• preserve retry behavior when both output fields are empty
• add regression coverage for the exact response shapes that repeatedly broke PR feat(mcp): support 2026-07-28 protocol #318's review workflow

Root cause

The configured deepseek-v4-flash endpoint completed the review but returned the JSON payload in message.reasoning_content while leaving message.content empty. The workflow only read content, retried three times, and failed before the Codex step could run.

Validation

• node --check .github/scripts/deepseek-common.mjs
• npx vitest run tests/deepseek-common.test.ts tests/codex-pr-review-context.test.ts (13 passed)
• npm run typecheck
• npm run lint (passes with 8 pre-existing warnings)

Bootstrap note

This PR fixes the bot that normally reviews PRs, so its own bot review must be skipped to avoid the same circular failure. After this lands, PR #318 will be rerun through the repaired bot workflow.