fix: azuread_privileged_access_group_eligibility_schedule resource is handled too many request

[kenchan0130]

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Description

This PR introduces a retry mechanism for the PrivilegedAccessGroupEligibilityScheduleResource during the read operation.
Previously, transient API failures could cause unexpected errors when retrieving the eligibility schedule.
By adding retry logic, this update improves reliability and reduces the likelihood of read failures caused by temporary service issues.

No breaking changes are expected from this update.

Below is the actual error log that occurred when running terraform plan on a resource using for_each.

Planning failed. Terraform encountered an error while generating this plan.
╷
│ Error: retrieving Identity Governance Privileged Access Group Eligibility Schedule (Privileged Access Group Eligibility Schedule: "a60f015a-f4d0-4afc-89a6-33b1477b01aa_member_e17c7a98-3926-4195-8d63-70024902e939"): unexpected status 429 (429 Too Many Requests) with error: UnknownError: Too Many Requests
│ 
│   with azuread_privileged_access_group_eligibility_schedule.this["20df8bc2-1012-40f2-a349-3f57ba7957ab"],
│   on main.tf line 1, in resource "azuread_privileged_access_group_eligibility_schedule" "this":
│   97: resource "azuread_privileged_access_group_eligibility_schedule" "this" {
│ 
│ retrieving Identity Governance Privileged Access Group Eligibility Schedule
│ (Privileged Access Group Eligibility Schedule:
│ "a60f015a-f4d0-4afc-89a6-33b1477b01aa_member_e17c7a98-3926-4195-8d63-70024902e939"):
│ unexpected status 429 (429 Too Many Requests) with error: UnknownError: Too
│ Many Requests
╵

Changes to existing Resource / Data Source

• I have added an explanation of what my changes do and why I'd like you to include them.
• I have written new tests for my resource or data source changes & updated any relevant documentation.
• I have successfully run tests with my changes locally.
• (For state migration changes only) I have manually tested any migration paths.

Testing

• My submission includes test coverage as described in the Contribution Guide.
• Test logs or evidence are included below (or an explanation is provided if testing evidence cannot be included).

As this update focuses on adding retry behavior for read operations, testing primarily involved validating successful retries under simulated transient failure conditions.

Change Log

• azuread_privileged_access_group_eligibility_schedule – added retry logic to improve reliability during read requests

Type of PR

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

Related Issue(s)

n/a

Rollback Plan

If needed, the change can be reverted by removing the retry mechanism from the read request handler.

Changes to Security Controls

This PR does not introduce any changes to security controls, such as access controls, encryption, or logging.