vpc_firewall_rules: fix provider produced invalid plan

[sudomateo]

Added acceptance tests to surface the issue reported in #453.

See #453 (comment) for an understanding of what's happening.

[sudomateo]

I updated the integration tests to showcase the issue.

> TEST_ACC_NAME=TestAccCloudResourceFirewallRules make testacc
-> Running terraform acceptance tests
=== RUN   TestAccCloudResourceFirewallRules_full
=== PAUSE TestAccCloudResourceFirewallRules_full
=== CONT  TestAccCloudResourceFirewallRules_full
    resource_vpc_firewall_rules_test.go:282: Step 3/4 error: Error running apply: exit status 1

        Error: Provider produced inconsistent result after apply

        When applying changes to
        oxide_vpc_firewall_rules.acc-firewall_rules-2d36a60b-ec4c-49a9-91d0-12d47d335477,
        provider "provider[\"registry.terraform.io/hashicorp/oxide\"]" produced an
        unexpected new value: .rules: planned set element
        cty.ObjectVal(map[string]cty.Value{"action":cty.StringVal("allow"),
        "description":cty.StringVal("Allow HTTPS."),
        "direction":cty.StringVal("inbound"),
        "filters":cty.ObjectVal(map[string]cty.Value{"hosts":cty.SetVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{"type":cty.StringVal("vpc"),
        "value":cty.StringVal("acc-terraform-912feb34-dff8-4a5b-9ffa-5440e2209a14")})}),
        "ports":cty.SetVal([]cty.Value{cty.StringVal("443")}),
        "protocols":cty.SetVal([]cty.Value{cty.StringVal("TCP")})}),
        "id":cty.StringVal("3e5d6309-9527-4e70-bca7-1b83d495bb92"),
        "name":cty.StringVal("allow-https"), "priority":cty.NumberIntVal(50),
        "status":cty.StringVal("enabled"),
        "targets":cty.SetVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{"type":cty.StringVal("subnet"),
        "value":cty.StringVal("acc-terraform-912feb34-dff8-4a5b-9ffa-5440e2209a14")})}),
        "time_created":cty.StringVal("2025-06-20 21:29:18.65834 +0000 UTC"),
        "time_modified":cty.StringVal("2025-06-20 21:29:18.65834 +0000 UTC")}) does
        not correlate with any element in actual.

        This is a bug in the provider, which should be reported in the provider's own
        issue tracker.
--- FAIL: TestAccCloudResourceFirewallRules_full (8.47s)
FAIL
FAIL    github.com/oxidecomputer/terraform-provider-oxide/internal/provider     8.475s
FAIL
make: *** [Makefile:82: testacc] Error 1

[sudomateo]

Superseded by #456.