Fix: Preserve HTTP headers in ParallelChannel sub-calls

[XiaoYingGee]

Problem

When using ParallelChannel, HTTP headers set on the parent controller (e.g., Authorization headers) are lost during sub-calls because they are not propagated to the sub-controllers.

Solution

• Propagate all HTTP headers from parent controller to each sub-controller
• Preserve application-set headers (like Authorization) on each sub-call
• Maintain backward compatibility