fix: keep streamed text when providers skip text-start

[tsubasakong]

Issue for this PR

Closes #5210

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Issue #5210 reports that some providers emit text-delta events without a preceding text-start, which caused streamed assistant text to be dropped instead of being attached to the current output part.

This PR creates the current assistant text part on the first text-delta when a provider skips text-start, keeps subsequent deltas/end metadata attached to that same part, and adds a focused regression test for the delta-first streaming path.

How did you verify your code works?

I ran git diff --cached --check locally and also attempted the focused regression test with bun test test/session/processor.test.ts --timeout 30000, but that broader run was blocked in this worktree because local dependencies such as zod and drizzle-orm were not installed.

Screenshots / recordings

Not applicable — this change is in the streaming/session processing path.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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