fix(sdk-react): clear stale stream values after joinStream history refresh

[pawel-twardziak]

Summary

This PR fixes a stale state issue in useStream/useStreamLGP after joinStream completes.

When joinStream succeeds and history is refetched, the hook could keep showing stale local stream
values if no final values event was applied.
This change clears local stream state after a successful history refresh so the hook falls back to the
latest /history values.

What Changed

• Updated libs/sdk/src/react/stream.lgp.tsx to reset local stream state (return null) after
  history.mutate(threadId) returns a latest head.
• Added a regression test in libs/sdk-validation/src/tests/stream.test.tsx covering:
  • stale local values after submit
  • joinStream path where no final values event is delivered
  • expected fallback to refreshed history values

Issue Fixed

Fixes a bug where joinStream could leave UI state stale (e.g. generating: true, outdated messages) even
after history was successfully refreshed.

Relevant Context

This is most visible in slow/interrupted stream flows where joinStream returns but no terminal values
event updates local stream state.

Testing

• Added integration-style regression test: joinStream clears stale values when history is refetched.

Fix #1969

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[Christian Bromann (christian-bromann)]

Thank you for raising the PR, we are currently working on new SDK packages to integrate in different frontend frameworks, see #2001. I will incorporate your PR in there manually.

[pawel-twardziak]

Hi Christian Bromann (@christian-bromann) thanks for that info, I forgot for a moment about that PR