fix(openapi): unit-returning handlers compile and document an empty body (closes #62)

[Kilerd]

Closes #62. Two problems with handlers that return nothing.

1. #[api] on a handler with no return type didn't compile

#[api(id = "no_return")]
async fn no_return() {}     // error[E0782]: expected a type, found a trait

The ReturnType::Default branch generated ( () as ::gotcha::Responsible) — a cast to a trait. So an ordinary "returns nothing" handler (a DELETE, say) could not be annotated at all. The existing no-args-handler test only covers no arguments; its handler returns String, so this path was never exercised.

Now emits <() as Responsible>::response().

2. () documented as "type": "void" — not a valid OpenAPI type

{"200":{"content":{"application/json":{"schema":{"type":"void"}}}}}

void isn't among the permitted values, so the document was invalid and client generators reject it. A new Schematic::empty_body() (default false, true for ()) lets the blanket Responsible impl document the unit type as a response with no content.

Which status? 200, not 204

Worth stating explicitly, since 204 No Content is the intuitive guess. axum sends 200 with an empty body:

impl IntoResponse for () { Body::empty().into_response() }   // axum-core/src/response/into_response.rs:126
impl IntoResponse for Body { Response::new(self) }           // :167

http::Response::new defaults to StatusCode::OK, and there is no NO_CONTENT anywhere in axum-core's response module. Documenting 204 would have recreated exactly the doc-vs-reality mismatch this PR is fixing. A handler that wants 204 returns StatusCode::NO_CONTENT explicitly.

The test pins this down by asserting the documented status against ().into_response().status() itself, so if axum ever changes, the test fails rather than the document silently lying.

Json<()> is deliberately left alone

Wrapping the unit in Json is genuinely different: it really does send null with a 200. That keeps its JSON response; only the schema changed, from the invalid "void" to an empty schema.

Test

tests/pass/openapi/unit_responses.rs covers all three shapes — no return type, explicit -> (), and Json<()> — asserting the response carries no content and that "void" appears nowhere.

Verified locally: gotcha (openapi) + gotcha_core suites, cargo clippy --all-features --workspace, cargo fmt --check.

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