ec2: group instances by launch spec and lifecycle in instant fleets

[nik-localstack]

EC2: create_fleet() (instant type) now groups instances sharing the same launch spec and lifecycle into a single response entry with aggregated InstanceIds, matching AWS behaviour.

Tested with https://github.com/localstack/localstack-pro/pull/6895

[viren-nadkarni]

With the v2026.04.0 release, we are no longer allowing any changes to Moto-Ext as it will break backwards compatibility. This change directly impacts the model for Fleet which is part of the persisted state.

[nik-localstack]

With the v2026.04.0 release, we are no longer allowing any changes to Moto-Ext as it will break backwards compatibility. This change directly impacts the model for Fleet which is part of the persisted state.

@viren-nadkarni If I'm not mistaken, this doesn't change how the fleet is stored but how it is represented in the response of the create fleet operation.
What it changes is only how the instances are grouped. The reason for this change is for better parity in aws validated responses of https://github.com/localstack/localstack-pro/pull/6895 .

I'm fine with skipping this change if you still have any concerns about state management.

[viren-nadkarni]

this doesn't change how the fleet is stored but how it is represented in the response of the create fleet operation

Effectively yes, but the logic resides in the Fleet model. For example: if a state is created in the v2026.04.0 release, this proposed fix goes out in v2026.05.0, and the persisted state is loaded in v2026.05.0, it will still return older response - because the model has the old logic.