fix: propagate headers to local subagents

[Akatuoro]

Fixes #121

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[hyperspace-pr-bot]

Summary

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Preserve authentication headers across local sub-agent calls

Category: Bug Fix 🐛

Summary

This PR fixes JWT/header propagation when an A2A agent invokes a local sub-agent, which then calls MCP tools or destination-backed services. Previously, spawned local sub-agent contexts did not retain request headers, causing downstream destination resolution and MCP calls to lose the user JWT and fail authorization.

Changes

• Pass cds.context.headers into local sub-agent cds.spawn(...) calls so spawned execution contexts preserve incoming request headers.
• Update Cloud SDK JWT retrieval in MCP and sub-agent destination handling to fall back to cds.context when cds.context.http.req is unavailable.
• Add a changelog entry for the header propagation fix.

Impact

JWTs can now flow from the main A2A service through local sub-agents and into destination-backed MCP tool invocations, supporting authenticated multi-agent setups deployed within a single CAP project.

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• Added relevant entry to the change log?

Fixes #121
Related issue: #121

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PR Bot Information

Version: 1.31.30

• LLM: gpt-5.5
• Summary Prompt: Default Prompt
• File Content Strategy: Full file content
• Output Template: Repository PR Template
• GithubContextProvider: [0.9.4: ] JWT gets lost in a setup of A2A orchestrator -> A2A Sub agents -> MCP server #121
• Event Trigger: pull_request.opened
• Correlation ID: 7ed604b0-b0cf-11f1-8bc1-02a19b14de64