Function buildSyntheticV3Capabilities

Build synthetic v3 capabilities when the agent advertises get_adcp_capabilities (a v3-only tool) but the call itself failed — either the executor threw, or the response was non-success with no structurally v3-shaped data (the case looksLikeV3Capabilities would have caught).

The agent is verifiably v3 because the v3-only discovery tool is present in tools/list. Falling back to v2 synthetic in this case triggers v2.5-schema lookups that have nothing to do with the original failure, cascading "AdCP schema data for version v2.5 not found" across every subsequent step. Treating as v3 (synthetic) lets the caller continue against the right adapter set; the underlying get_adcp_capabilities failure still surfaces in logs at the call site.

Sets _synthetic: true so version-compat checks know detail-level fields (idempotency TTL, supportedVersions) are unknown — callers skip those checks rather than throw VersionUnsupportedError on a verifiably-v3 agent.

Refs: #1217 (carve-out from #1197/closed), #1189 / #1201 (the v3-shape heuristic this complements).