fix: pass security credentials to external A2A sub-agents

[2862282695gjh-afk]

Problem

When a Nexent agent calls an external A2A sub-agent via the SDK execution path (ExternalA2AAgentProxy), the configured security credentials are not sent in the request headers, causing 403 Forbidden from external agents that require authentication (e.g., X-HW-ID + X-HW-APPKEY, Bearer JWT).

Root Cause

The backend a2a_client_service.call_agent already handles security credentials via _build_security_request_parts. However, the SDK execution path (managed agents, used when a local agent calls an external A2A sub-agent) goes through ExternalA2AAgentProxy that only supports a simple Bearer token (api_key) — it never receives the configured security_credentials.

Data chain broken at 4 levels:

1. query_external_sub_agents does not return security_credentials
2. _build_external_agent_config does not read/pass credentials
3. ExternalA2AAgentConfig / A2AAgentInfo have no security headers field
4. _build_headers ignores securitySchemes

Fix (4 files, +90 lines)

backend/database/a2a_agent_db.py (+3 lines)

query_external_sub_agents returns security_schemes, security_requirements, security_credentials.

backend/agents/create_agent_info.py (+38 lines)

New _build_security_headers(agent):

• Reads security_schemes + security_requirements + security_credentials
• Builds headers per A2A securitySchemes spec:
  • apiKeySecurityScheme (header) → headers[name] = credential
  • httpAuthSecurityScheme (bearer/basic) → Authorization: "<scheme> <credential>"
• Supports wrapper + flat scheme format, case-insensitive location
• _build_external_agent_config passes result as custom_headers

sdk/nexent/core/agents/agent_model.py (+6 lines)

• ExternalA2AAgentConfig adds custom_headers field
• to_a2a_agent_info() passes custom_headers

sdk/nexent/core/agents/a2a_agent_proxy.py (+4 lines)

• A2AAgentInfo adds custom_headers field
• _build_headers() merges custom_headers

Supported Auth

• apiKey in header: any name (X-HW-ID, X-HW-APPKEY, etc.) ✅
• httpAuth: Bearer/basic/JWT → Authorization ✅
• Mixed combinations ✅
• Wrapper + flat scheme formats ✅

Tests

test/backend/agents/test_a2a_security_headers.py:

• apiKey double-header (X-HW-ID + X-HW-APPKEY)
• httpAuth Bearer JWT
• No credentials → empty

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