TPT-4294: terraform: Update linode_instance resource to support reserved IP assignment at creation

[mgwoj]

📝 Description

Extend the linode_instance resource to support assigning a pre-reserved IPv4 address to the instance at creation time. Currently, reserved IP assignment after creation is handled by linode_instance_reserved_ip_assignment. This story adds support for specifying a reserved IP at the point of POST /linode/instances.

✔️ How to Test

make TEST_SUITE="instance" test-int

[Copilot]

Pull request overview

Updates the linode_instance resource implementation and tests to better support reserved IPv4 assignment workflows by normalizing how instance IPv4 addresses are flattened into Terraform state and ensuring reserved-IP acceptance tests execute as part of the standard instance suite.

Changes:

• Refactors repeated IPv4 flattening logic into a shared flattenInstanceIPv4 helper.
• Updates instance read/create paths to use the shared IPv4 flattening helper.
• Removes opt-in gating from reserved-IP acceptance tests so they run under the normal instance integration tag.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
linode/instance/resource.go	Uses shared IPv4 flattening helper when setting ipv4 in state during read/create.
linode/instance/flatten.go	Introduces flattenInstanceIPv4 helper and reuses it in flattenInstance.
linode/instance/flatten_unit_test.go	Adds unit coverage for the new IPv4 flattening helper.
linode/instance/resource_test.go	Ensures reserved-IP acceptance tests run without opt-in gating.

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[dawiddzhafarov]

Sorry but I don't quite understand those changes - what changed that it enables using reserved IP in this resource?
To me, the source code is exactly the same, the difference is that some part of it was extracted to a func and tests were added for this func so there are no changes in the behavior of the resource - or am I missing something?

[mgwoj]

Sorry but I don't quite understand those changes - what changed that it enables using reserved IP in this resource? To me, the source code is exactly the same, the difference is that some part of it was extracted to a func and tests were added for this func so there are no changes in the behavior of the resource - or am I missing something?

It is only cleanup for the functionality which was already introduced earlier.

[dawiddzhafarov]

Okay, so the PR title and commit message are a bit misleading for me

[lgarber-akamai]

Looks great, nice work!