#1915 fix(playground): handle legacy string message content

Summary

• Playgrounds saved before the AI SDK v5 migration store message content as a plain string, but transformFromLegacy assumed an array and called .map on it.
• Opening any such playground crashed with message.content.map is not a function.
• Fix: normalize non-array (legacy string / nullish) content into a single text part before mapping, so the editor and the v4→v5 part upgrade always see an array.

Fixes #1905

Verification

$ npx tsx --test tests/playground-transform-legacy.test.ts
# tests 3
# pass 3
# fail 0

• New regression test covers legacy string content, preserved array content (v4 tool-call/tool-result upgrade), and nullish content.
• tsc --noEmit clean on changed files.

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[!NOTE]
Low Risk
Localized playground load-path fix with regression tests; no auth, persistence, or API contract changes.

Overview
Fixes a crash when opening playgrounds saved before the AI SDK v5 migration, where message content was stored as a plain string (or null) instead of a parts array. transformFromLegacy called .map on that value and threw message.content.map is not a function.

The change introduces toContentParts, which leaves array content unchanged and wraps non-array values in a single { type: "text", text: ... } part (empty string for null/undefined). transformFromLegacy runs that normalization before the existing v4→v5 part upgrade logic.

Adds playground-transform-legacy.test.ts covering legacy string content, array + tool-call/result upgrade, and nullish content.

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