feat(client): update client headers partially rather than overwriting

[0x676e67]

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[Copilot]

Pull Request Overview

This PR updates the client header logic to merge new header values into existing ones rather than replacing them entirely. Key changes include:

• Renaming the variable from default_headers to updated_headers in context for clarity.
• Switching from a full header swap to iterating over new headers and inserting them individually.
• Updating inline comments to reflect the new partial update behavior.

Comments suppressed due to low confidence (1)

src/async_impl/client.rs:799

• Changing the header update logic from swapping the entire headers object to iterating and inserting could lead to unintended reordering or duplicate header issues if duplicate keys exist. Please verify that the new merge approach correctly preserves the intended header ordering and handling of duplicate keys.

update = update.headers(|default_headers| {