Fix execution order of `read_before_serialization` and `modify_before_serialization` (#3798)

## Motivation and Context
Fixes the execution order of
[modify_before_serialization](https://docs.rs/aws-smithy-runtime-api/latest/aws_smithy_runtime_api/client/interceptors/trait.Intercept.html#method.modify_before_serialization)
and
[read_before_serialization](https://docs.rs/aws-smithy-runtime-api/latest/aws_smithy_runtime_api/client/interceptors/trait.Intercept.html#method.read_before_serialization)
in the orchestrator.

## Description
The fix ensures that the execution order of these interceptor methods
aligns with our orchestrator design. While customers may see a behavior
change as a result, we don't treat this as a new `BehaviorVersion` since
it is intended as a bug fix.

## Testing
Existing tests in CI

## Checklist
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [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.

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