Fix nullable UUID type mismatch in Go code generator

[nogates]

Summary

• Nullable UUID fields (format: uuid + nullable: true) were being mapped to NullableString in formatter.py, causing generated getter/setter methods to return *string while the method signatures expected uuid.UUID — resulting in compile errors.
• Added a dedicated NullableUUID struct to utils.j2 (matching the pattern of NullableBool, NullableString, NullableTime, etc.).
• Updated formatter.py line 158 to emit NullableUUID / NewNullableUUID for nullable uuid fields instead of NullableString.

Example compile errors this fixes (from datadog-api-spec PR #5821 CI run):

model_incident_rule_data_attributes_request.go:191: cannot use *o.IncidentTypeUuid.Get() (variable of type string) as uuid.UUID value in return statement
model_incident_rule_data_attributes_request.go:201: cannot use o.IncidentTypeUuid.Get() (value of type *string) as *uuid.UUID value in return statement
model_incident_rule_data_attributes_request.go:211: cannot use &v (value of type *uuid.UUID) as *string value in argument to o.IncidentTypeUuid.Set

Test plan

• Regenerate client with a spec that contains a nullable uuid field and confirm the generated code compiles without type errors.
• Confirm NullableUUID marshal/unmarshal round-trips correctly (JSON UUID strings ↔ uuid.UUID).

🤖 Generated with Claude Code

[datadog-prod-us1-3]

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