Emit the envelope-level adcp_version echo on get_adcp_capabilities

[numarasSigmaSoftware]

Context

Flagged in the #1546 re-review: the capabilities response omits the envelope-level adcp_version echo. The echo is advisory at AdCP 3.1 but becomes a blocking conformance check at the 3.2 cut, and the change is protocol-wide (every transport's capabilities envelope), so it warrants its own change rather than riding an unrelated PR.

Scope

• Add the adcp_version envelope echo to the get_adcp_capabilities response across MCP/A2A/REST.
• Ground the exact field placement in the pinned spec's envelope schema (dist/schemas/<pinned>/) before implementation, per the Spec-Grounding Gate.
• Grade it on the captured wire per transport (not reconstructed responses).