Fix error struct with default impl

[codypenta]

Motivation and Context

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

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

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

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

    @required
    message: String

    code: String = "400"

    context: String
}

Testing

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

Checklist

• 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

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[codypenta]

@jdisanti let me know if I need to make any changes, pretty new to the code base 😄

[jdisanti]

Thanks for taking the time to do this! ❤️

Just a couple minor things.

[jdisanti]

Thanks for thinking to add a test!

It would be nice for this to get its own isolated test case though so that it doesn't get lost in the future.

Would you mind creating a ProtocolParserGeneratorTest.kt file in src/test/kotlin/... same path as ProtocolParserGenerator .../ that tests this in isolation? Here's an example of how to do a client integration test that runs the full client code generator against a test model and verifies it compiles:

smithy-rs/codegen-client/src/test/kotlin/software/amazon/smithy/rust/codegen/client/smithy/protocols/AwsQueryTest.kt

Line 12 in c4a14d7

class AwsQueryTest {

[codypenta]

Sure can, would you like me to also revert the changes made in the current file or should I leave that in?

[jdisanti]

Yeah, please revert.

[codypenta]

Done and done 😄