Land #4551 (RSC prerender cache empty-payload fix): resolve test threads, merge, cherry-pick to release/17.0.0

[justin808]

Goal

Land PR #4551 ([Pro] Fix RSC payload prerender cache storing an empty payload) — the server half of shakacode/react_on_rails_rsc#187 (infinite RSC payload retry loop) — and cherry-pick it to release/17.0.0.

Locked decisions

• Implementation is approved as-is (CodeRabbit approved, CI green). No implementation changes — only the three open review threads, all test-level:
  1. Ihab: the request spec (react_on_rails_pro/spec/dummy/spec/requests/rsc_payload_spec.rb) never reaches the cached response — the dummy generates a fresh CSP nonce per request, which changes the prerender cache key, so both calls render through Node. Make the second request actually hit the cache (pin/stub the nonce or use a cache key the test controls), and assert the cached path.
  2. Ihab: pin the chunk.dup behavior directly in a helper spec (react_on_rails_pro_helper.rb) so the endpoint test still fails if the code regresses to chunk.delete("html").
  3. Greptile P2: cover the second cache hit in the destructive-consumer replay test (stream_cache_spec.rb).
• Merge to main via merge queue, then cherry-pick to release/17.0.0 (confirmed RC-line stabilizer: wrong-output bug — an empty payload gets cached and served).

Scope / collision set

Ruby only: react_on_rails_pro/lib/react_on_rails_pro/stream_cache.rb, react_on_rails_pro/app/helpers/react_on_rails_pro_helper.rb, specs, CHANGELOG.md (collides with every other item — rebase last-merged).

Acceptance criteria

• All three review threads addressed and resolved; Ihab re-review or maintainer dismissal.
• cd react_on_rails_pro && bundle exec rspec spec/react_on_rails_pro/stream_cache_spec.rb and the dummy request spec green locally; bundle exec rubocop clean.
• Merged to main; cherry-pick commit on release/17.0.0.
• After BOTH this and the [Pro] Keep RSC payload retries inside one cached promise #4564 item land: manually close Infinite RSC payload retry loop when using webpack bundler react_on_rails_rsc#187 (cross-repo, will not auto-close) referencing both PRs.

Dependencies

None (fully parallel with the other items except the shared CHANGELOG.md line).

[justin808]

Done: #4551 merged to main (176dc8a) and cherry-picked to release/17.0.0 via #4589 (c16b4bd, patch-ID-verified). The remaining cross-repo step - closing shakacode/react_on_rails_rsc#187 - is coordinated under #4567 and the #4569 assembly sequence once the client-half backport (#4594) lands.