docs(agents): add A2A agent authentication headers guide
New page: docs/a2a_agent_headers.md
Covers all three header forwarding methods:
- Static headers (admin-configured, always sent)
- Forward client headers (extra_headers — admin lists names, client supplies values)
- Convention-based x-a2a-{agent_name/id}-{header} (no admin config needed)
Documents merge precedence (static wins), header isolation guarantee,
combining all three methods, and API reference for static_headers / extra_headers fields.
Registered in sidebars.js under /a2a - A2A Agent Gateway.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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