Add external_status_code support to internal provider resource

[ne-sachirou]

Added support for external_status_code in external monitor in #258 . However, the implementation was missing and the test failed like https://github.com/mackerelio-labs/terraform-provider-mackerel/actions/runs/14212042760/job/39821559851 .

Output from acceptance testing:

$ make testacc TESTS=TestAccCompat_MackerelMonitorResource
=== RUN   TestAccCompat_MackerelMonitorResource
=== PAUSE TestAccCompat_MackerelMonitorResource
=== CONT  TestAccCompat_MackerelMonitorResource
=== RUN   TestAccCompat_MackerelMonitorResource/host_metric/undefined
=== PAUSE TestAccCompat_MackerelMonitorResource/host_metric/undefined
=== RUN   TestAccCompat_MackerelMonitorResource/connectivity/undefined
=== PAUSE TestAccCompat_MackerelMonitorResource/connectivity/undefined
=== RUN   TestAccCompat_MackerelMonitorResource/connectivity/null
=== PAUSE TestAccCompat_MackerelMonitorResource/connectivity/null
=== RUN   TestAccCompat_MackerelMonitorResource/external
=== PAUSE TestAccCompat_MackerelMonitorResource/external
=== RUN   TestAccCompat_MackerelMonitorResource/expression
=== PAUSE TestAccCompat_MackerelMonitorResource/expression
=== RUN   TestAccCompat_MackerelMonitorResource/query
=== PAUSE TestAccCompat_MackerelMonitorResource/query
=== RUN   TestAccCompat_MackerelMonitorResource/host_metric/null
=== PAUSE TestAccCompat_MackerelMonitorResource/host_metric/null
=== RUN   TestAccCompat_MackerelMonitorResource/host_metric/empty
=== PAUSE TestAccCompat_MackerelMonitorResource/host_metric/empty
=== RUN   TestAccCompat_MackerelMonitorResource/connectivity/empty
=== PAUSE TestAccCompat_MackerelMonitorResource/connectivity/empty
=== RUN   TestAccCompat_MackerelMonitorResource/service_metric
=== PAUSE TestAccCompat_MackerelMonitorResource/service_metric
=== RUN   TestAccCompat_MackerelMonitorResource/external/null
=== PAUSE TestAccCompat_MackerelMonitorResource/external/null
=== RUN   TestAccCompat_MackerelMonitorResource/external/empty
=== PAUSE TestAccCompat_MackerelMonitorResource/external/empty
=== RUN   TestAccCompat_MackerelMonitorResource/anomaly_detection
=== PAUSE TestAccCompat_MackerelMonitorResource/anomaly_detection
=== CONT  TestAccCompat_MackerelMonitorResource/host_metric/undefined
=== CONT  TestAccCompat_MackerelMonitorResource/host_metric/empty
=== CONT  TestAccCompat_MackerelMonitorResource/external/null
=== CONT  TestAccCompat_MackerelMonitorResource/service_metric
=== CONT  TestAccCompat_MackerelMonitorResource/connectivity/empty
=== CONT  TestAccCompat_MackerelMonitorResource/expression
=== CONT  TestAccCompat_MackerelMonitorResource/anomaly_detection
=== CONT  TestAccCompat_MackerelMonitorResource/connectivity/null
=== CONT  TestAccCompat_MackerelMonitorResource/host_metric/null
=== CONT  TestAccCompat_MackerelMonitorResource/connectivity/undefined
=== CONT  TestAccCompat_MackerelMonitorResource/external
=== CONT  TestAccCompat_MackerelMonitorResource/query
=== CONT  TestAccCompat_MackerelMonitorResource/external/empty
--- PASS: TestAccCompat_MackerelMonitorResource (0.00s)
    --- PASS: TestAccCompat_MackerelMonitorResource/external (4.25s)
    --- PASS: TestAccCompat_MackerelMonitorResource/expression (4.30s)
    --- PASS: TestAccCompat_MackerelMonitorResource/external/null (4.32s)
    --- PASS: TestAccCompat_MackerelMonitorResource/connectivity/empty (4.41s)
    --- PASS: TestAccCompat_MackerelMonitorResource/host_metric/undefined (4.43s)
    --- PASS: TestAccCompat_MackerelMonitorResource/host_metric/null (4.43s)
    --- PASS: TestAccCompat_MackerelMonitorResource/connectivity/null (4.46s)
    --- PASS: TestAccCompat_MackerelMonitorResource/host_metric/empty (4.48s)
    --- PASS: TestAccCompat_MackerelMonitorResource/connectivity/undefined (4.50s)
    --- PASS: TestAccCompat_MackerelMonitorResource/query (4.58s)
    --- PASS: TestAccCompat_MackerelMonitorResource/service_metric (4.74s)
    --- PASS: TestAccCompat_MackerelMonitorResource/anomaly_detection (5.07s)
    --- PASS: TestAccCompat_MackerelMonitorResource/external/empty (2.91s)
PASS

$ make testacc TESTS=TestAccMackerelMonitor_External
=== RUN   TestAccMackerelMonitor_External
=== PAUSE TestAccMackerelMonitor_External
=== CONT  TestAccMackerelMonitor_External
--- PASS: TestAccMackerelMonitor_External (3.55s)
PASS

$ make testacc TESTS=TestAccDataSourceMackerelMonitorExternal
=== RUN   TestAccDataSourceMackerelMonitorExternal
=== PAUSE TestAccDataSourceMackerelMonitorExternal
=== CONT  TestAccDataSourceMackerelMonitorExternal
--- PASS: TestAccDataSourceMackerelMonitorExternal (1.94s)
PASS