Check Content-Type header in all server protocols (#2531)

## Checklist
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rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates

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codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/protocols/ServerHttpBoundProtocolGenerator.kt

@@ -57,7 +57,6 @@ import software.amazon.smithy.rust.codegen.core.smithy.protocols.HttpBoundProtoc
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.HttpLocation
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.Protocol
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.ProtocolFunctions
-import software.amazon.smithy.rust.codegen.core.smithy.protocols.RestJson
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.parse.StructuredDataParserGenerator
 import software.amazon.smithy.rust.codegen.core.smithy.traits.SyntheticInputTrait
 import software.amazon.smithy.rust.codegen.core.smithy.transformers.operationErrors
@@ -234,6 +233,11 @@ class ServerHttpBoundProtocolTraitImplGenerator(
                 rustTemplate(init, *codegenScope)
             }
         }
+        // This checks for the expected `Content-Type` header if the `@httpPayload` trait is present, as dictated by
+        // the core Smithy library, which _does not_ require deserializing the payload.
+        // If no members have `@httpPayload`, the expected `Content-Type` header as dictated _by the protocol_ is
+        // checked later on for non-streaming operations, in `serverRenderShapeParser`: that check _does_ require at
+        // least buffering the entire payload, since the check must only be performed if the payload is empty.
         val verifyRequestContentTypeHeader = writable {
             operationShape
                 .inputShape(model)
@@ -242,11 +246,15 @@ class ServerHttpBoundProtocolTraitImplGenerator(
                 ?.let { payload ->
                     val target = model.expectShape(payload.target)
                     if (!target.isBlobShape || target.hasTrait<MediaTypeTrait>()) {
-                        val expectedRequestContentType = httpBindingResolver.requestContentType(operationShape)
-                            ?.let { "Some(${it.dq()})" } ?: "None"
+                        // `null` is only returned by Smithy when there are no members, but we know there's at least
+                        // the one with `@httpPayload`, so `!!` is safe here.
+                        val expectedRequestContentType = httpBindingResolver.requestContentType(operationShape)!!
                         rustTemplate(
                             """
-                            #{SmithyHttpServer}::protocol::content_type_header_classifier(request.headers(), $expectedRequestContentType)?;
+                            #{SmithyHttpServer}::protocol::content_type_header_classifier(
+                                request.headers(),
+                                Some("$expectedRequestContentType"),
+                            )?;
                             """,
                             *codegenScope,
                         )
@@ -689,30 +697,25 @@ class ServerHttpBoundProtocolTraitImplGenerator(
         rust("let (parts, body) = request.into_parts();")
         val parser = structuredDataParser.serverInputParser(operationShape)
         val noInputs = model.expectShape(operationShape.inputShape).expectTrait<SyntheticInputTrait>().originalId == null
+
         if (parser != null) {
-            rustTemplate(
-                """
-                let bytes = #{Hyper}::body::to_bytes(body).await?;
-                if !bytes.is_empty() {
-                """,
-                *codegenScope,
-            )
-            if (protocol is RestJson) {
+            // `null` is only returned by Smithy when there are no members, but we know there's at least one, since
+            // there's something to parse (i.e. `parser != null`), so `!!` is safe here.
+            val expectedRequestContentType = httpBindingResolver.requestContentType(operationShape)!!
+            rustTemplate("let bytes = #{Hyper}::body::to_bytes(body).await?;", *codegenScope)
+            rustBlock("if !bytes.is_empty()") {
                 rustTemplate(
                     """
-                    #{SmithyHttpServer}::protocol::content_type_header_classifier(&parts.headers, Some("application/json"))?;
+                    #{SmithyHttpServer}::protocol::content_type_header_classifier(
+                        &parts.headers, 
+                        Some("$expectedRequestContentType"),
+                    )?;
+                    input = #{parser}(bytes.as_ref(), input)?;
                     """,
                     *codegenScope,
+                    "parser" to parser,
                 )
             }
-            rustTemplate(
-                """
-                input = #{parser}(bytes.as_ref(), input)?;
-                }
-                """,
-                *codegenScope,
-                "parser" to parser,
-            )
         }
         for (binding in bindings) {
             val member = binding.member

rust-runtime/aws-smithy-http-server/src/protocol/rest_json_1/rejection.rs

@@ -115,6 +115,8 @@ pub enum RequestRejection {
     NotAcceptable,
 
     /// Used when checking the `Content-Type` header.
+    /// This is bubbled up in the generated SDK when calling
+    /// [`crate::protocol::content_type_header_classifier`] in `from_request`.
     #[error("expected `Content-Type` header not found: {0}")]
     MissingContentType(#[from] MissingContentTypeReason),
 

rust-runtime/aws-smithy-http-server/src/protocol/rest_json_1/runtime_error.rs

@@ -47,6 +47,9 @@ pub enum RuntimeError {
     InternalFailure(crate::Error),
     /// Request contained an `Accept` header with a MIME type, and the server cannot return a response
     /// body adhering to that MIME type.
+    /// This is returned directly (i.e. without going through a [`RequestRejection`] first) in the
+    /// generated SDK when calling [`crate::protocol::accept_header_classifier`] in
+    /// `from_request`.
     NotAcceptable,
     /// The request does not contain the expected `Content-Type` header value.
     UnsupportedMediaType,