Fix: Preserve Authorization headers in fetch instrumentation (#208)

* Fix: Preserve Authorization headers in fetch instrumentation

The OpenTelemetry fetch instrumentation was overwriting existing headers
when adding trace context. This caused issues with third-party SDKs like
E2B that set Authorization headers, resulting in 'authorization header is
missing' errors.

Changes:
- Only add trace context headers if they don't already exist
- Prevents overwriting critical headers like Authorization
- Properly preserves all existing headers from init.headers

* Create release changeset

* recommended fix

Author: jhaynie

CHANGELOG.md

@@ -1,5 +1,11 @@
 # @agentuity/sdk Changelog
 
+## 0.0.158
+
+### Patch Changes
+
+- Add fix for otel headers not correctly preserving original outgoing headers on fetch
+
 ## 0.0.157
 
 ### Patch Changes