`thar-be-registries`: `BTreeMap` key sorting breaks endpoint order in hosts.toml

### Package I'm using:

`thar-be-registries` in `bottlerocket-core-kit`

### What I expected to happen:

When configuring `settings.container-registry.mirrors` with multiple endpoints, the generated `/etc/containerd/certs.d//hosts.toml` should preserve the endpoint order as specified. Since containerd uses section order as priority (first = primary, rest = fallback), endpoints should appear in `hosts.toml` in the same order they were configured.

### What actually happened:

The endpoint order in `hosts.toml` does not match the configured order — endpoints are sorted alphabetically instead. This causes containerd to use an unintended endpoint as primary.

Verified on a live node:
- `/etc/containerd/thar-be-registries.toml` — endpoint order is **correct** (matches configuration)
- `/etc/containerd/certs.d/docker.io/hosts.toml` — endpoint order is **wrong** (sorted alphabetically)

```toml
# thar-be-registries.toml (CORRECT order)
[[mirrors]]
registry = "docker.io"
endpoint = [
"https://mirror-a.example.com",
"https://mirror-b.example.com"

# /etc/containerd/certs.d/docker.io/hosts.toml (WRONG order — sorted alphabetically)
server = "https://registry-1.docker.io"

[host."https://mirror-b.example.com"]
capabilities = ["pull", "resolve"]

[host."https://mirror-a.example.com"]
capabilities = ["pull", "resolve"]
```

### How to reproduce the problem:
Configure two or more mirror endpoints whose URLs sort in a different alphabetical order than the intended priority order. Inspect the generated `hosts.toml` — endpoints will appear in alphabetical order regardless of the configured order.