fix: add automatic text-start and text-end events

[titanism2197]

Automatically emit text-start and text-end events for proper stream lifecycle management.
Solves the "text part text-0 not found" error by eliminating the need for manual stream wrapping.

Problem

Users encountered "text part text-0 not found" errors in production:

data: {"type":"start"}
data: {"type":"start-step"}
data: {"type":"text-delta","id":"text-0","delta":"지"}
data: {"type":"error","errorText":"text part text-0 not found"}

Root Cause: text-delta events were emitted without preceding text-start events, causing the AI SDK to fail when tracking text parts.

Previous Workaround: Users had to manually wrap streams:

// ❌ Manual wrapper required
const ensureTextParts = (stream) => {
  // Manually inject text-start/text-end events
};

Solution

Text lifecycle events are now emitted automatically following AI SDK standards:

// ✅ Now works automatically!
const result = await streamText({
  model: helicone('gpt-4o'),
  prompt: 'Hello'
});
// text-start → text-delta → text-end → finish

Changes

• Add text part state tracking (textStarted, textEnded)
• Auto-emit text-start before first text-delta
• Auto-emit text-end when finish_reason arrives or stream ends
• Prevent duplicate events with Set tracking
• Update tool-calling test to expect text-start as first event

Event Flow

Before (Error):
  text-delta (id: text-0) ❌ <- Missing text-start!
    → ERROR: "text part text-0 not found"

After (Fixed):
  text-start (id: text-0) ✅
    → text-delta (id: text-0) ✅
    → text-delta (id: text-0) ✅
    → text-end (id: text-0) ✅
    → finish ✅

All tests passing: 77/77 ✅

Files Changed

• src/helicone-language-model.ts (+36/-1 lines)
• src/tests/tool-calling.test.ts (+2/-2 lines)

Total: 38 insertions(+), 3 deletions(-) ✨

Breaking Changes

⚠️ Minor: Tests expecting text-delta as first event need to expect text-start.
This matches AI SDK's expected event flow and is the correct behavior.

Migration

No code changes needed for end users. Events are added automatically.
If you were using a custom ensureTextParts wrapper, you can now remove it!

Fixes

• Resolves "text part text-0 not found" errors
• Ensures proper stream lifecycle
• Matches Vercel AI SDK event standards

[cyborgshead]

@juliettech13 please have a look, need to fix broken streams