fix: preserve multimodal tool results across client round trips

[sravan-v]

🎯 Changes

Fixes #1283.

TOOL_CALL_RESULT arrives with the tool output serialized as a string. When that string contains a valid ContentPart[], keeping it as plain text means the structured result is lost before uiMessagesToWire builds the next request.

This change restores valid content-part arrays while leaving ordinary string and JSON tool results unchanged. It also adds regression coverage for the stream-processor → wire round trip, including the e2e suite.

No docs change: this restores the existing multimodal tool-result behavior rather than adding a new API.

✅ Checklist

• I followed the contributing guide
• I tested locally with pnpm run test:pr
• I fully understand the code in this pull request, including code generated with AI assistance
• I updated docs/ for this change, or this change is not user-facing
• I added a changeset for the published package change

🚀 Release Impact

• This affects published code and includes a changeset
• This is docs/CI/dev-only

Local pnpm test:pr was not run in this environment; the PR checks are expected to validate the test, typecheck, lint, and build paths.

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