Fix: surface mid-stream OAuth errors as login chip

[annzhang-db]

Summary

• Mid-stream errors (e.g. MCP connection OAuth failures) were forwarded to the client as raw { type: 'error', errorText } chunks
• The client's message rendering pipeline only processes data-error parts, so these errors were logged but never displayed
• Converts mid-stream error chunks to { type: 'data-error', data: errorText } so isCredentialErrorMessage can match them and MessageOAuthError renders the login chip

Root cause

stream-fallback.ts had one code path that correctly emitted data-error (in fallbackToGenerateText) but the mid-stream error forwarding path wrote the raw AI SDK error chunk instead. The client ignores type: 'error' chunks entirely.

Test plan

• Trigger an OAuth-gated MCP tool (e.g. a UC connection requiring GitHub login) — the login chip should appear instead of a silent failure
• Non-OAuth mid-stream errors should still appear as error messages via the existing MessageError component

This pull request and its description were written by Claude.