Follow-up to SERVER-125656, requested in DRIVERS-3541.

SERVER-125656 added structured diagnostics (failingPath, underlyingCode, underlyingReason, failingElement) to the writeError for 2dsphere key-extraction failures, but they serialize at the top level of the error. Per the CRUD spec, drivers set details from writeErrors[].errInfo, so spec-following drivers drop these top-level fields and users never see them.

We should move the four fields into a nested errInfo sub-object so drivers surface them as details with no driver-side change, matching the existing document-validation failure pattern.

Note that this only changes the client-facing writeError wire shape. The operator-facing validate diagnostics are built from the typed accessors, not the wire format, so they are unaffected.