fix: preserve active UI message parts across merged stream step boundaries (#19324)

## Background

Merged UI streams could abort and lose valid continuation text when one
source finished a step while another source still had an active text or
reasoning part.

## Root Cause

The UI stream processor unconditionally replaced its global active text
and reasoning maps on every finish-step. The reproduction confirmed that
another merged source's subsequent valid delta was then reported as
missing.

`WorkflowChatTransport` also mirrored that old behavior in its stream
normalizer, so changing only the core consumer could split an
interleaved part into multiple UI parts and leave the first one stuck in
the `streaming` state.

## Summary

- Keep active text and reasoning parts open until their explicit end
chunks.
- Align workflow stream normalization with the new lifetime model by
preserving open IDs across `finish-step` while releasing ended IDs for
legitimate reuse in later steps.
- Add patch changesets for both `ai` and `@ai-sdk/workflow`.
- Strengthen workflow compatibility tests to assert exact part
boundaries and completion states.

## Testing

- Added core regression coverage for text and reasoning parts continuing
across interleaved step boundaries.
- Updated workflow stream tests to verify error-free, lossless
consumption as a single completed part.
- Verified well-formed multi-step streams can still reuse part ID `"0"`
after the prior part ends.
- `pnpm --filter ai exec vitest --config vitest.node.config.js --run
src/ui/process-ui-message-stream.test.ts`
- `pnpm --filter @ai-sdk/workflow test:node`
- `pnpm --filter @ai-sdk/workflow test:edge`
- `pnpm check`
- `pnpm type-check:full`

## End-to-end Validation

- `pnpm -C examples/ai-functions exec tsx
src/reproduction/issue-19321-merged-finish-step.ts` — completed
successfully with `first half second half` and no missing-part error.

## Related Issues

Fixes #19321

Closes #19323

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: tpsdeveloper13 <197825877+tpsdeveloper13@users.noreply.github.com>
Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>