Fix presigned put bug with flex checksums (#3971)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Fixing bug reported in
awslabs/aws-sdk-rust#1240 (comment)

## Description
<!--- Describe your changes in detail -->

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Updated presigning integ tests to account for behavior specified in SEP,
added new test for the user provided checksum case

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

.changelog/presigning-bugfix.md

@@ -0,0 +1,10 @@
+---
+applies_to: ["aws-sdk-rust"]
+authors: ["landonxjames"]
+references: ["aws-sdk-rust#1240"]
+breaking: false
+new_feature: false
+bug_fix: true
+---
+
+Fix bug with presigned requests introduced by new flexibile checksums functionality

aws/rust-runtime/aws-inlineable/src/http_request_checksum.rs

@@ -193,24 +193,22 @@ where
             .load::<RequestChecksumCalculation>()
             .unwrap_or(&RequestChecksumCalculation::WhenSupported);
 
-        // Determine if we actually calculate the checksum. If the user setting is WhenSupported (the default)
-        // we always calculate it (because this interceptor isn't added if it isn't supported). If it is
-        // WhenRequired we only calculate it if the checksum is marked required on the trait.
-        let calculate_checksum = match request_checksum_calculation {
-            RequestChecksumCalculation::WhenRequired => request_checksum_required,
-            RequestChecksumCalculation::WhenSupported => true,
+        // Need to know if this is a presigned req because we do not calculate checksums for those.
+        let is_presigned_req = cfg.load::<PresigningMarker>().is_some();
+
+        // Determine if we actually calculate the checksum. If this is a presigned request we do not
+        // If the user setting is WhenSupported (the default) we always calculate it (because this interceptor
+        // isn't added if it isn't supported). If it is WhenRequired we only calculate it if the checksum
+        // is marked required on the trait.
+        let calculate_checksum = match (request_checksum_calculation, is_presigned_req) {
+            (_, true) => false,
+            (RequestChecksumCalculation::WhenRequired, false) => request_checksum_required,
+            (RequestChecksumCalculation::WhenSupported, false) => true,
             _ => true,
         };
 
         // Calculate the checksum if necessary
         if calculate_checksum {
-            let is_presigned_req = cfg.load::<PresigningMarker>().is_some();
-
-            // If this is a presigned request and the user has not set a checksum we short circuit
-            if is_presigned_req && checksum_algorithm.is_none() {
-                return Ok(());
-            }
-
             // If a checksum override is set in the ConfigBag we use that instead (currently only used by S3Express)
             // If we have made it this far without a checksum being set we set the default (currently Crc32)
             let checksum_algorithm =

aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/HttpRequestChecksumDecorator.kt

@@ -119,8 +119,8 @@ private fun HttpChecksumTrait.checksumAlgorithmToStr(
             // Checksums are required but a user can't set one, so we set crc32 for them
             rust("""let checksum_algorithm = Some("crc32");""")
         } else {
-            // Use checksum algo set by user or crc32 if one has not been set
-            rust("""let checksum_algorithm = checksum_algorithm.map(|algorithm| algorithm.as_str()).or(Some("crc32"));""")
+            // Use checksum algo set by user
+            rust("""let checksum_algorithm = checksum_algorithm.map(|algorithm| algorithm.as_str());""")
         }
 
         // If a request checksum is not required and there's no way to set one, do nothing
@@ -205,17 +205,22 @@ class HttpRequestChecksumCustomization(
                                     // From the httpChecksum trait
                                     let http_checksum_required = $requestChecksumRequired;
 
+                                    let is_presigned_req = cfg.load::<#{PresigningMarker}>().is_some();
+
+                                    // If the request is presigned we do not set a default.
                                     // If the RequestChecksumCalculation is WhenSupported and the user has not set a checksum value or algo
                                     // we default to Crc32. If it is WhenRequired and a checksum is required by the trait and the user has not
                                     // set a checksum value or algo we also set the default. In all other cases we do nothing.
                                     match (
                                         request_checksum_calculation,
                                         http_checksum_required,
                                         user_set_checksum_value,
-                                        user_set_checksum_algo
+                                        user_set_checksum_algo,
+                                        is_presigned_req,
                                     ) {
-                                        (#{RequestChecksumCalculation}::WhenSupported, _, false, false)
-                                        | (#{RequestChecksumCalculation}::WhenRequired, true, false, false) => {
+                                        (_, _, _, _, true) => {}
+                                        (#{RequestChecksumCalculation}::WhenSupported, _, false, false, _)
+                                        | (#{RequestChecksumCalculation}::WhenRequired, true, false, false, _) => {
                                             request.headers_mut().insert(${requestAlgoHeader.dq()}, "CRC32");
                                         }
                                         _ => {},
@@ -254,6 +259,7 @@ class HttpRequestChecksumCustomization(
                                             codegenContext.model,
                                         ),
                                     ),
+                                "PresigningMarker" to AwsRuntimeType.presigning().resolve("PresigningMarker"),
                             )
                         }
                     }