Add inbound_auth passthrough for Skills Sandbox.

execute_skills now reads the AgentKit SDK captured inbound_auth credential
and forwards it as an inbound_auth header when calling the Skills Sandbox.
This lets sandbox agents and downstream MCP tools consume the original inbound
auth credential without additional token exchange.
This also adds redacted debug logging before sandbox dispatch, reporting only
presence, length, and sha256 digest, plus focused coverage for inbound_auth
passthrough and empty credential behavior.

Validation:
Ran targeted Skills Sandbox tests with agentkit-sdk-python==0.8.3 and
a2a-sdk==0.3.15.

9 sample contract tests passed.
End to End User -- Runtime -- Skill Sandbox -- MCP tool test passed.