fix(api): omit nullable references from the schemas that require them

[DutchyD]

Follows #38. Found by generating a client from the published document and parsing a real /v1/diagnostics response through it:

lastRefresh.attemptedAt -> Invalid ISO datetime
diagnostics.0.position -> expected object, received undefined

The second one is ours. #38 added a transformer that drops nullable properties from required, because the serializer omits them rather than writing null. It read nullability off the generated schema — which works for string? ("type": ["null","string"]) but not for a nullable reference:

"lastRefresh": { "oneOf": [ { "type": "null" }, { "$ref": "#/components/schemas/RefreshView" } ] }

That form carries no type of its own, so the check missed it and both properties stayed required while the API omits them. Any consumer validating against the document rejected a perfectly valid response.

Nullability now comes from the property via NullabilityInfoContext, which is the same thing the serializer consults, so the schema cannot disagree with what is written regardless of how the schema spells the type.

Result

nullable properties still required: NONE
enum properties: 12
schemas: 21

The document diff is exactly two entries — DiagnosticView.position and GetDiagnosticsResponse.lastRefresh.

(The datetime failure is not a defect here: the API emits +00:00, which format: date-time permits. The consumer's generated schema was too strict and is fixed on that side.)

Verification

dotnet build 0 warnings, dotnet format --verify-no-changes clean, 364/364 tests pass. The regenerated document is committed, so CI's drift check covers it.