[simplifier] Go 1.26 modernization and cleanup in planner and dump packages

Code merged in the last 24 hours (PRs #2201, #2208) introduced or
preserved patterns that can be simplified using Go 1.26 stdlib and
general cleanup. All items below are mechanical, behavior-preserving
changes.

---

### 1. Replace `compareStringSlices` with `slices.Equal`

**File:** `internal/declarative/planner/event_gateway_backend_cluster_planner.go` lines 480-489

The manual loop is equivalent to `slices.Equal(a, b)`. Per AGENTS.md
Go 1.26 modernization standards, the `slices` package should be used
instead of manual loops.

```go
// Before
func compareStringSlices(a, b []string) bool {
if len(a) != len(b) {
return false
}
for i := range a {
if a[i] != b[i] {
return false
}
}
return true
}

// After: inline slices.Equal at call sites and remove the function
// Callers in event_gateway_listener_planner.go and
// event_gateway_backend_cluster_planner.go become:
// slices.Equal(current.Addresses, desired.Addresses)
```

**Callers (3):**
- `event_gateway_backend_cluster_planner.go:365`
- `event_gateway_listener_planner.go:476`
- `event_gateway_listener_planner.go:488`

---

### 2. Replace `compareStringMaps` with `maps.Equal`

**File:** `internal/declarative/planner/event_gateway_backend_cluster_planner.go` lines 512-522

Same rationale. The manual map comparison is `maps.Equal(a, b)`.

```go
// Before
func compareStringMaps(a, b map[string]string) bool { ... }

// After: inline maps.Equal at call sites and remove the function
```

**Callers (2):**
- `event_gateway_backend_cluster_planner.go:406`
- `event_gateway_static_key_planner.go:292`

---

### 3. Remove redundant `reflect.Kind` case listing in `isEmptyValue`

**File:** `internal/cmd/root/verbs/dump/declarative.go` lines 967-986

The exhaustive listing of `reflect.Invalid`, `reflect.Bool`, ...,
`reflect.UnsafePointer` all return `false`, which is identical to the
`return false` default at line 989. The 17-line case block can be
deleted with no behavior change.

---

### 4. Hoist compiled regex out of `isValidHostname`

**File:** `internal/declarative/resources/portal_custom_domain.go` lines 137-143

`regexp.MustCompile` is called on every invocation. Move the compiled
regex to a package-level variable so it is compiled once at init time.

```go
// Before
func isValidHostname(hostname string) bool {
pattern := `^...`
hostnameRegex := regexp.MustCompile(pattern)
return hostnameRegex.MatchString(hostname)
}

// After
var hostnameRegex = regexp.MustCompile(
`^([a-zA-Z0-9]([a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?\.)*` +
`[a-zA-Z0-9]([a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?$`,
)

func isValidHostname(hostname string) bool {
return hostnameRegex.MatchString(hostname)
}
```

---

### Validation steps

After applying changes, run:

```sh
go fix ./...
make format
make build
make lint
make test
```

> Generated by [Code Simplifier](https://github.com/Kong/kongctl/actions/runs/35300741437) · opus46 · 236.5 AIC · ⌖ 29.4 AIC · ⊞ 5.9K · [◷](https://github.com/search?q=repo%3AKong%2Fkongctl+is%3Aissue+%22gh-aw-workflow-call-id%3A+Kong%2Fkongctl%2Fcode-simplifier%22&type=issues)
>

_{Add this agentic workflow to your repo}

To install this agentic workflow, run

```
gh aw add githubnext/agentics/workflows/code-simplifier.md@eb7950f37d350af6fa09d19827c4883e72947221
```

> - [x] expires on Sep 23, 2026, 2:57 AM UTC

Start with the listed planner files, internal/cmd/root/verbs/dump/declarative.go, and internal/declarative/resources/portal_custom_domain.go; read the named functions and their callers. Apply the specified Go 1.26 simplifications while preserving behavior, then run go fix ./..., make format, make build, make lint, and make test. Done means all four cleanup items are addressed and validation passes.

Written by the indexing model from the issue text.