fix: lazily create text/reasoning parts when resumed stream starts with delta

[MaxwellCalkin]

Summary

Fixes #13160

When useChat resumeStream resumes a UI stream after a disconnect, the resumed stream may start with text-delta (or reasoning-delta) chunks for text/reasoning parts whose text-start/reasoning-start was already sent before the disconnect. The SDK currently throws a UIMessageStreamError because state.activeTextParts[chunk.id] is undefined.

This PR changes the text-delta, text-end, reasoning-delta, and reasoning-end handlers in processUIMessageStream to lazily create the missing part instead of throwing. When a delta/end chunk arrives for an unknown part ID, a new empty part is initialized in the active parts map and pushed to state.message.parts, then processing continues normally. This makes stream resumption work seamlessly.

Changes

• text-delta: If state.activeTextParts[chunk.id] is null, create a new TextUIPart with empty text in streaming state, register it, then append the delta
• text-end: Same lazy creation, then immediately mark as done
• reasoning-delta: If state.activeReasoningParts[chunk.id] is null, create a new ReasoningUIPart with empty text in streaming state, register it, then append the delta
• reasoning-end: Same lazy creation, then immediately mark as done

Tests updated

The existing tests that expected UIMessageStreamError throws for these cases have been updated to verify the lazy creation behavior instead, with proper delta accumulation and end-state assertions.

Test plan

• All 93 existing tests in process-ui-message-stream.test.ts pass
• Updated tests verify text-delta without text-start creates part and accumulates text correctly
• Updated tests verify reasoning-delta without reasoning-start creates part and accumulates text correctly
• Updated tests verify text-end/reasoning-end without start creates part and marks as done
• Manual: verify useChat resumeStream works when resumed stream starts with text-delta for an existing part

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