feat: add ACLP monitor list entities method

[shkaruna]

📝 Description

1. Update ACLP monitor AlertDefinition struct. Add scope & region fields.
2. Add entity envelope in AlertDefinition for GET, POST and PUT API responses.

`"entities":` { // empty dict for region and account scope alert
"url":"/monitor/services/{service_type}/alert-definitions/{id}/entities",
 "count": integer,
 "has_more_resources": boolean
   }

3. Add new method to list entities.
   API: GET /monitor/services/{service_type}/alert-definitions/{id}/entities
4. Add unit & integration tests.

JIRA: DPS-41869

✔️ How to Test

What are the steps to reproduce the issue or verify the changes?
Not Applicable

How do I run the relevant unit/integration tests?

Prerequisites
Go 1.19+ installed
Valid Linode API token with monitor permissions
Export LINODE_TOKEN environment variable for integration tests

Unit tests:

1. Run all the unit tests:
   make test-unit
2. Run Monitor alert definition unit tests:
   go test -v ./test/unit/... -run MonitorAlertDefinition
   Output:
   === RUN TestCreateMonitorAlertDefinition
   --- PASS: TestCreateMonitorAlertDefinition (0.00s)
   === RUN TestCreateMonitorAlertDefinitionWithIdempotency
   --- PASS: TestCreateMonitorAlertDefinitionWithIdempotency (0.00s)
   === RUN TestGetMonitorAlertDefinition
   --- PASS: TestGetMonitorAlertDefinition (0.00s)
   === RUN TestListMonitorAlertDefinitions
   --- PASS: TestListMonitorAlertDefinitions (0.00s)
   === RUN TestUpdateMonitorAlertDefinition
   --- PASS: TestUpdateMonitorAlertDefinition (0.00s)
   === RUN TestUpdateMonitorAlertDefinition_LabelOnly
   --- PASS: TestUpdateMonitorAlertDefinition_LabelOnly (0.00s)
   === RUN TestDeleteMonitorAlertDefinition
   --- PASS: TestDeleteMonitorAlertDefinition (0.00s)
   === RUN TestListMonitorAlertDefinitionEntities
   --- PASS: TestListMonitorAlertDefinitionEntities (0.00s)
   PASS
   ok github.com/linode/linodego/test/unit (cached)

Integration tests:
Verified with Production PAT & devcloud PAT.

1. Play:
   make test-int
   make test-int TEST_ARGS='-run "Alert"'
   cd test && make test-int TEST_TIMEOUT=5h
   make[1]: Entering directory '/home/shkaruna/sdk/linodego/test'
   2026/04/08 12:08:47 [INFO] LINODE_FIXTURE_MODE play will be used for tests
   2026/04/08 12:08:47 [INFO] Fixture mode play - Test Linode Cloud Firewall not created
   === RUN TestMonitorAlertDefinition_smoke
   --- PASS: TestMonitorAlertDefinition_smoke (0.00s)
   === RUN TestMonitorAlertDefinitions_List
   --- PASS: TestMonitorAlertDefinitions_List (0.00s)
   === RUN TestMonitorAlertChannels_List
   --- PASS: TestMonitorAlertChannels_List (0.00s)
   === RUN TestMonitorAlertDefinition_CreateWithIdempotency
   --- PASS: TestMonitorAlertDefinition_CreateWithIdempotency (0.00s)
   === RUN TestMonitorAlertDefinitionEntities_List
   --- PASS: TestMonitorAlertDefinitionEntities_List (0.00s)
   PASS
   ok github.com/linode/linodego/test/integration 0.015s
   make[1]: Leaving directory '/home/shkaruna/sdk/linodego/test'

2. Record
   export LINODE_TOKEN="Linode API token"
   ENABLE_CLOUD_FW=false make TEST_ARGS='-run "Alert"' fixtures

[shkaruna]

@satkumar-akamai : Please review

[Copilot]

Pull request overview

Adds support for listing entities associated with an ACLP monitor alert definition and updates the alert definition model/tests to reflect new API fields.

Changes:

• Extended AlertDefinition to include scope, regions, and an entities envelope.
• Added a new client method to fetch the paginated entities list for an alert definition.
• Updated unit tests, integration tests, and integration fixtures to match the new/updated API shapes.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
monitor_alert_definitions.go	Adds new scope/entities types & fields and introduces an entities-list client method.
test/unit/monitor_alert_definitions_test.go	Updates mocked API payloads and assertions; adds a unit test for listing entities.
test/integration/monitor_alert_definitions_test.go	Adds a (currently skipped) integration test stub for listing alert definition entities.
test/integration/fixtures/TestMonitorAlertDefinition.yaml	Updates recorded fixture interactions/responses for monitor alert definition integration coverage.

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[yec-akamai]

Unit test passed. will run integration test once API in prod

[yec-akamai]

Can you address the conflict before merging?