fix: change Explode field from bool to *bool for proper serialization

[curiosity-the-rover]

Problem

When Explode is bool with omitempty JSON/YAML tag, false values are not serialized to the output. This prevents users from explicitly setting explode: false in OpenAPI specs.

For example:

type Parameter struct {
    Explode bool `json:"explode,omitempty" yaml:"explode,omitempty"`
}

p := Parameter{Explode: false}
yaml.Marshal(p) // Output: {} - Explode field is missing!

Solution

Change Explode from bool to *bool in:

• Parameter struct (parameter.go)
• Header struct (header.go)
• Encoding struct (encoding.go)

This allows:

• nil → omit the field (use default behavior)
• &true → serialize as explode: true
• &false → serialize as explode: false

The builder methods are updated to take address of the value.

Changes

• parameter.go: Explode bool → Explode *bool, builder updated
• header.go: Explode bool → Explode *bool, builder updated
• encoding.go: Explode bool → Explode *bool, builder updated

Testing

All existing tests pass.

Related

This change is needed to support explicit explode: false generation in tools like swaggo/swag.