azurerm_mssql_managed_instance fix identity bug and updating subnet_id

[wyattfry]

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Description

Fixes a bug that introduced a discrepancy between Azure's API behavior and the provider's behavior, namely, Azure allows MSSQL managed instances to have either a System Assigned or User Assigned identity, or both. The provider only allowed for one or the other.

Also, it seems that subnet_id may have been immutable but is now mutable, and thus must be included in the Update command. This PR adds subnet_id to the command.

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Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• azurerm_mssql_managed_instance - fix identity and subnet_id bugs [azurerm_mssql_managed_instance fix identity bug and updating subnet_id #28319]

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)

Fixes #28320
Fixes #21214
Fixes #19802
Fixes https://hashicorp.atlassian.net/browse/IPL-6520

Note

If this PR changes meaningfully during the course of review please update the title and description as required.

[wyattfry]

1 failing acceptance test, due to timeout, rerunning that one test

        API Response:
        ----[start]----
        {"name":"31930286-4dff-48ff-a098-15853e0712d0","status":"Failed","startTime":"2024-12-20T17:23:44.937Z","error":{"code":"ConflictingServerOperation","message":"Server
        'acctestsqlserver241220165729791029' is busy with another operation. Please
        try your operation later."}}
        -----[end]-----
--- FAIL: TestAccMsSqlManagedInstance_identity (4482.45s)

[wyattfry]

because subnet_id has since become mutable (idk about the admin creds, but the op fails without them), it must now be included in resource updates.

[wyattfry]

1 failing acceptance test, due to timeout, rerunning that one test

        API Response:
        ----[start]----
        {"name":"31930286-4dff-48ff-a098-15853e0712d0","status":"Failed","startTime":"2024-12-20T17:23:44.937Z","error":{"code":"ConflictingServerOperation","message":"Server
        'acctestsqlserver241220165729791029' is busy with another operation. Please
        try your operation later."}}
        -----[end]-----
--- FAIL: TestAccMsSqlManagedInstance_identity (4482.45s)

Test succeeded on the first retry

[catriona-m]

Thanks @wyattfry - I left a couple of minor comments inline but otherwise this is looking pretty good 😄

[catriona-m]

Thanks @wyattfry LGTM ✨