fix(gen): omit unset flattened-oneOf struct fields from requests

[UlricQin]

Problem

DashboardVariable is one flattened struct covering the datasource, custom and query arms. Only the query arm carries variable_query, but the generated tag was:

VariableQuery DashboardVariableQuery `json:"variable_query,omitempty"`

,omitempty never drops a struct value, so every datasource or custom variable was sent as:

{"kind": "datasource", "name": "metrics_ds", "variable_query": {"kind": ""}}

The API validates each arm strictly, so it rejected /monit/dashboard/create, /monit/dashboard/update and /monit/dashboard/panel/preview with unknown field "variable_query". That covers any dashboard that has a datasource or custom variable.

Cause

isStructSchema resolves a $ref by looking at the raw target schema. For DashboardVariableQuery that schema is a oneOf, so the function returned false. emitModels, however, emits a named oneOf of object arms as one flattened struct (resolveObject → mergeAllOf). Because the field was misclassified, it skipped the existing ,omitzero rule for optional struct-typed request fields.

Fix

Classify the $ref target the same way emitModels emits it: run it through resolveObject first. Inline oneOf schemas still return false; goTypeOf emits those as any.

Regenerating changes exactly one line in models_gen.go: the variable_query tag goes from omitempty to omitzero. No types change and nothing becomes a pointer, so this is not a breaking change.

Tests

• TestEmitStructOptionalFlattenedOneOfRequestFieldOmitsZero (generator): an optional request field whose $ref target is a oneOf of objects is emitted with ,omitzero.
• TestDashboardVariableOmitsUnsetVariableQuery (SDK): a datasource variable marshals without variable_query, and a query variable still includes it.

Both tests fail on main and pass with this change. go generate ./... reproduces the committed output, golangci-lint run reports 0 issues, and go test ./... passes.

I also built the CLI against this branch and ran it end to end against a test account, using a dashboard with a datasource variable. Panel preview, create, get and delete all succeeded, and the stored variable has no variable_query key.