Stop modifying user-provided headers silently

[bledden]

Summary

Fixes #300

Previously, when a user specified a custom user-agent header, the SDK would silently append its version string to it. For example:

const openrouter = createOpenRouter({
  headers: { 'user-agent': 'my-custom-app/1.0' }
});
// Actual header sent: "my-custom-app/1.0 ai-sdk/openrouter/1.5.4"

This unexpected modification could break header-based authentication or routing logic.

Changes

The new behavior respects user-provided headers:

• If NO user-agent is provided: SDK sets user-agent: ai-sdk/openrouter/X.X.X (same as before)
• If user-agent IS provided: User's header is preserved unchanged, and SDK version is sent via X-OpenRouter-SDK-Version header instead

Validation

Verified that OpenRouter's API works fine without the SDK version in user-agent:

curl -s "https://openrouter.ai/api/v1/chat/completions" \
  -H "User-Agent: my-custom-app/1.0" \
  ...
# Request succeeds - OpenRouter doesn't require SDK version in user-agent

Test Plan

• Added unit tests for withUserAgentSuffix covering all scenarios
• All 133 unit tests pass (node + edge)
• E2E tests pass
• Build succeeds

🤖 Generated with Claude Code