Fix SigV4 signature for URIs with literal query parameters

[Alan4506]

Problem

When generating a client for Amazon Q Business using smithy-python, the ChatSync operation fails with InvalidSignatureException. The same request succeeds with boto3 using identical credentials and endpoint.

The root cause is that ChatSync has a literal query parameter in its Smithy HTTP trait:

"smithy.api#http": {
    "uri": "/applications/{applicationId}/conversations?sync",
    "method": "POST"
}

Link to the model file: https://github.com/aws/api-models-aws/blob/9eafbc359b1c6a7187ac921009b9eef85de71d91/models/qbusiness/service/2023-11-27/qbusiness-2023-11-27.json#L2244-L2250

The ?sync query parameter (no = sign, no value) is dropped during SigV4 canonical query string computation. parse_qsl("sync") returns [] because it ignores keys without values by default. The server includes sync= in its canonical query string, so the signatures don't match.

Fix

Add keep_blank_values=True to both sync and async _format_canonical_query methods. This makes parse_qsl("sync", keep_blank_values=True) return [('sync', '')], which correctly produces sync= in the canonical query string.

Testing

1. Generate a Q Business client using smithy-python
2. Create a Q Business anonymous application and pass the application id to an environment variable:

export QBUSINESS_APPLICATION_ID=<application-id>

3. Run a test like the following:

async def test_chat_sync():
    client = QBusinessClient(config=Config(
        endpoint_uri="https://qbusiness.us-east-1.api.aws",
        region="us-east-1",
        aws_credentials_identity_resolver=EnvironmentCredentialsResolver(),
    ))
    response = await client.chat_sync(ChatSyncInput(
        application_id=APPLICATION_ID,
        user_message="Hello",
        client_token=str(uuid.uuid4()),
    ))

4. Without this fix: InvalidSignatureException
5. With this fix: 200 OK with valid response

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[jonathan343]

Looks good, thanks @Alan4506!

Can you add some unit tests in tests/unit/test_signers.py for this case to prevent any regressions? It would look something like below:

Note: Both TestSigV4Signer and TestAsyncSigV4Signer already exist.

class TestSigV4Signer:
    SIGV4_SYNC_SIGNER = SigV4Signer()

    def test_format_canonical_query_keeps_blank_values(self) -> None:
        canonical_query = self.SIGV4_SYNC_SIGNER._format_canonical_query(
            query="foo=bar&baz="
    )

    assert canonical_query == "foo=bar&baz="

class TestAsyncSigV4Signer:
    SIGV4_ASYNC_SIGNER = AsyncSigV4Signer()

    async def test_format_canonical_query_keeps_blank_values(self) -> None:
        canonical_query = await self.SIGV4_ASYNC_SIGNER._format_canonical_query(
            query="foo=bar&baz="
    )

    assert canonical_query == "foo=bar&baz="

[jonathan343]

LGTM. Thanks @Alan4506!