Add API Gateway Accept header customization

[Alan4506]

Context

While generating an API Gateway client from its model locally and running integration tests, several operations failed to deserialize their responses. For example, CreateRestApi returns a createdDate member modeled as a timestamp (epoch-seconds under restJson1), but the response returned an ISO-8601 string, so timestamp deserialization failed:

smithy_json._private.deserializers.JSONTokenError: Error parsing JSON.
Expected token of type `number` at path `createdDate`, but found: `string`: 2026-06-10T17:17:25Z

The root cause is API Gateway specific. Unless the client sends Accept: application/json, API Gateway responds with application/hal+json, in which timestamps are serialized as ISO-8601 strings instead of the epoch-seconds that the restJson1 protocol expects.

This is a documented API Gateway client requirement, not general protocol behavior. The Smithy spec calls it out as a service-specific customization: Amazon API Gateway Customizations. Other Smithy SDKs already implement this same customization in their codegen (e.g. smithy-rs, smithy-go, smithy-kotlin). This change adds the equivalent to smithy-python.

Changes

• Runtime interceptor (smithy-aws-core): a new ApiGatewayAcceptHeaderInterceptor that sets Accept: application/json on every request in modify_before_signing.
• Codegen integration (codegen/aws/core): a new ApiGatewayIntegration, registered with the PythonIntegration SPI. When the service being generated is API Gateway, it emits a plugin that registers the interceptor on the client; for all other services it does nothing. It lives under a new customizations/apigateway/ package, leaving room for the other service customizations the Smithy spec lists (e.g. Glacier, S3, Machine Learning).

Generated clients for the API Gateway service now include an accept_header.py plugin that appends the interceptor.

Testing

• New unit tests (smithy-aws-core): verify the interceptor sets Accept: application/json, and that it overrides any pre-existing Accept header. Both pass.
• Existing unit tests in smithy-aws-core still pass.
• Regenerated the API Gateway client and confirmed the generated client.py registers the accept_header_plugin and that the plugin appends ApiGatewayAcceptHeaderInterceptor.
• With the customization in place, the following script (AWS credentials must be set up first) can run without deserialization issue:

import asyncio

from aws_sdk_apigateway.client import APIGatewayClient
from aws_sdk_apigateway.config import Config
from aws_sdk_apigateway.models import CreateRestApiInput, DeleteRestApiInput
from smithy_aws_core.identity import EnvironmentCredentialsResolver

REGION = "us-east-1"


async def main() -> None:
    client = APIGatewayClient(
        config=Config(
            endpoint_uri=f"https://apigateway.{REGION}.amazonaws.com",
            region=REGION,
            aws_credentials_identity_resolver=EnvironmentCredentialsResolver(),
        )
    )

    # The response includes the `createdDate` timestamp member, which is what
    # fails to deserialize when API Gateway returns application/hal+json.
    created = await client.create_rest_api(
        input=CreateRestApiInput(name="accept-header-demo")
    )
    print(created.id, created.created_date)

    await client.delete_rest_api(input=DeleteRestApiInput(rest_api_id=created.id))


asyncio.run(main())

Without the customization this raises JSONTokenError: Expected token of type 'number' at path 'createdDate', but found: 'string'.

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