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

[Copilot]

Pull Request Overview

This PR fixes an issue where HTTP headers set on parent controllers are lost when making sub-calls through ParallelChannel. The fix ensures that application-set headers like Authorization are properly propagated to each sub-controller.

• Adds header propagation logic to preserve parent controller headers in sub-calls
• Maintains backward compatibility by only copying headers when they exist
• Improves functionality for applications that rely on HTTP headers for authentication/authorization

[Copilot]

[nitpick] The check for empty headers is redundant since the for loop will naturally handle empty header collections. This condition can be removed to simplify the code.

Suggested change

	if (parent_hdr.HeaderBegin() != parent_hdr.HeaderEnd()) {
	auto& sub_hdr = d->sub_done(i)->cntl.http_request();
	for (auto it = parent_hdr.HeaderBegin(); it != parent_hdr.HeaderEnd(); ++it) {
	sub_hdr.AppendHeader(it->first, it->second);
	}
	auto& sub_hdr = d->sub_done(i)->cntl.http_request();
	for (auto it = parent_hdr.HeaderBegin(); it != parent_hdr.HeaderEnd(); ++it) {
	sub_hdr.AppendHeader(it->first, it->second);

[Copilot]

Using AppendHeader may result in duplicate headers if the sub-controller already has headers with the same name. Consider using SetHeader instead to avoid potential header duplication, or check if the header already exists before appending.

Suggested change

	sub_hdr.AppendHeader(it->first, it->second);
	sub_hdr.SetHeader(it->first, it->second);

[wwbmmm]

Does it need to check if sub-controller headers has exists key before AppendHeader or SetHeader? Avoid overwrite existing header.