fix: list_accounts not advertised in capabilities tool output

[KonstantinMirin]

Summary

list_accounts is fully implemented and registered with MCP and A2A transports, but src/core/tools/capabilities.py does not mention accounts in its output. The AdCP spec supports capability advertisement — buyer agents querying capabilities should see that this seller supports account management.

Follow-up from: #1011

Acceptance criteria

• capabilities tool output includes list_accounts (and sync_accounts if applicable)
• Verify the capability advertisement aligns with AdCP spec format
• Unit test covers accounts appearing in capabilities output