Fix error struct with default impl (#3190)

## 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 -->

When an error member has a default value, it will generate errors
(specifically is_none not found on type String) with smithy-rs client
unless you manually specify @required

More Here: #3182

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

Adds recomendation change to address error structures with a default
implementation like so:

```kotlin
if (errorMessageMember != null) {
    val symbol = symbolProvider.toSymbol(errorMessageMember)
    if (symbol.isOptional()) {
        rust(
            """
            if tmp.message.is_none() {
                tmp.message = _error_message;
            }
            """,
        )
    }
}
..... 
```


```smithy
@error("client")
structure Error {
    @required
    requestId: String

    @required
    message: String

    code: String = "400"

    context: String
}
```

## 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. -->

Added the above and ran `./gradlew codegen-client-test:build` with a
build successful

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

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

---------

Co-authored-by: John DiSanti <[email protected]>

CHANGELOG.next.toml

@@ -11,6 +11,14 @@
 # meta = { "breaking" = false, "tada" = false, "bug" = false, "target" = "client | server | all"}
 # author = "rcoh"
 
+[[smithy-rs]]
+message = """
+Fix rendering of @error structs when fields have default values
+"""
+references = ["smithy-rs#3182"]
+meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "client"}
+author = "codypenta"
+
 [[aws-sdk-rust]]
 message = "Change `ByteStream::into_async_read` to return `AsyncBufRead`"
 references = ["smithy-rs#3164"]

codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/protocol/ProtocolParserGenerator.kt

@@ -27,6 +27,7 @@ import software.amazon.smithy.rust.codegen.core.smithy.RuntimeType
 import software.amazon.smithy.rust.codegen.core.smithy.RustSymbolProvider
 import software.amazon.smithy.rust.codegen.core.smithy.customize.writeCustomizations
 import software.amazon.smithy.rust.codegen.core.smithy.generators.setterName
+import software.amazon.smithy.rust.codegen.core.smithy.isOptional
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.HttpBindingDescriptor
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.HttpLocation
 import software.amazon.smithy.rust.codegen.core.smithy.protocols.Protocol
@@ -163,7 +164,8 @@ class ProtocolParserGenerator(
                                 val errorMessageMember = errorShape.errorMessageMember()
                                 // If the message member is optional and wasn't set, we set a generic error message.
                                 if (errorMessageMember != null) {
-                                    if (errorMessageMember.isOptional) {
+                                    val symbol = symbolProvider.toSymbol(errorMessageMember)
+                                    if (symbol.isOptional()) {
                                         rust(
                                             """
                                             if tmp.message.is_none() {

codegen-client/src/test/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/protocol/ProtocolParserGeneratorTest.kt

@@ -0,0 +1,71 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.rust.codegen.client.smithy.generators.protocol
+
+import org.junit.jupiter.api.Test
+import software.amazon.smithy.rust.codegen.client.testutil.clientIntegrationTest
+import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
+
+class ProtocolParserGeneratorTest {
+    private val model = """
+        ${'$'}version: "2.0"
+        namespace test
+        
+        use aws.protocols#restJson1
+        
+        @restJson1
+        service TestService {
+            version: "2019-12-16",
+            operations: [SomeOperation]
+            errors: [SomeTopLevelError]
+        }
+        
+        @http(uri: "/SomeOperation", method: "POST")
+        operation SomeOperation {
+            input: SomeOperationInputOutput,
+            output: SomeOperationInputOutput,
+            errors: [SomeOperationError]
+        }
+
+        structure SomeOperationInputOutput {
+            payload: String,
+            a: String,
+            b: Integer
+        }
+        
+        @error("server")
+        structure SomeTopLevelError {
+            @required
+            requestId: String
+
+            @required
+            message: String
+
+            code: String = "400"
+
+            context: String
+        }
+        
+        @error("client")
+        structure SomeOperationError {
+            @required
+            requestId: String
+
+            @required
+            message: String
+
+            code: String = "400"
+
+            context: String
+        }
+    """
+        .asSmithyModel()
+
+    @Test
+    fun `generate an complex error structure that compiles`() {
+        clientIntegrationTest(model) { _, _ -> }
+    }
+}