fix(generator): an operation declaring security: [] still got the credential

[giraffesyo]

Closes #105.

security:
  - bearer: []
paths:
  /public:
    get:
      security: []      # explicitly public

authorization headers: ["Bearer s3cr3t" "Bearer s3cr3t"]

Auth was applied in do for every request, and no per-operation security reached the IR at all. A bearer token went to an endpoint the spec documents as needing none, landing in that server's logs, and an unauthenticated endpoint is the one most likely to be served by something other than the API itself.

authorization headers: ["Bearer s3cr3t" ""]

Absent and empty are different declarations

libopenapi distinguishes them, so the analyzer can too: an absent security field arrives as nil and inherits the document's, an empty one arrives as a non-nil empty slice and overrides it. Only the second opts out. An operation naming a scheme explicitly still authenticates.

do takes an authenticated argument rather than reading anything global, so the decision is made per operation at generation time and is visible in the generated call.

Scope

Choosing between several schemes per operation, or requiring several at once, is deliberately untouched. The client holds one AuthProvider, and honoring per-operation selection means either several providers or a provider that knows which scheme it satisfies, which is a design question the opt-out does not depend on. ir.OperationDef.SecurityReqs remains unpopulated, as it was.

Tests

• internal/analyzer/security_test.go: an operation that declares nothing inherits, one with security: [] opts out, one naming a scheme does not.
• internal/generator/e2e_operation_security_test.go: compiles and runs against httptest, checking the inheriting operation still sends Bearer s3cr3t while the public one sends nothing, that opting out changes nothing else about the request (the body still goes), and that a client with no provider is unaffected.

gofmt, go vet ./..., and go test ./... pass.