Adds CreatedAt to AgentPool

[ctrombley]

Description

This PR adds CreatedAt field to AgentPool struct, originally by @jpadrianoGo (#1150).

Testing plan

1. Generate the required environment variables for go test, TFE_ADDRESS and TFE_TOKEN
2. Run TFE_ADDRESS="https://example" TFE_TOKEN="example" go test ./... -v -run TestAgentPoolsReadCreatedAt. The new tests should pass.
3. CreatedAt is read for AgentPool.

{
  "data": {
    "id": "apool-***",
    "type": "agent-pools",
    "attributes": {
      "name": "cool-pool",
      "created-at": "2025-06-01T03:35:11.339Z",
      "agent-count": 0,
      "organization-scoped": true
    },
    "relationships": {
      "organization": {
        "data": {
          "id": "orgTest",
          "type": "organizations"
        }
      },
      "agents": {
        "links": {
          "related": "/api/v2/agent-pools/apool-***/agents"
        }
      },
      "authentication-tokens": {
        "links": {
          "related": "/api/v2/agent-pools/apool-***/authentication-tokens"
        }
      },
      "oauth-clients": {
        "links": {
          "related": "/api/v2/organizations/orgTest/oauth-clients?filter[agent_pool]=apool-***"
        }
      },
      "workspaces": {
        "data": []
      },
      "allowed-workspaces": {
        "data": []
      }
    },
    "links": {
      "self": "/api/v2/agent-pools/apool-***"
    }
  }
}

External links

Output from tests

Including output from tests may require access to a TFE instance. Ignore this section if you have no environment to test against.

$ TFE_ADDRESS="https://example" TFE_TOKEN="example"go test ./... -v -run TestAgentPoolsReadCreatedAt

=== RUN   TestAgentPoolsReadCreatedAt
--- PASS: TestAgentPoolsReadCreatedAt (3.00s)
PASS
ok      github.com/hashicorp/go-tfe     3.067s

[datadog-terraform-cloud-hashicorp]

Datadog Report

Branch report: agentPool/CreatedAt
Commit report: cf79ce7
Test service: hashicorp/go-tfe

✅ 0 Failed, 1473 Passed, 186 Skipped, 32m 3.1s Total Time
➡️ Test Sessions change in coverage: 1 no change

[Maed223]

Nice and simple 👍

[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.

[jpadrianoGo]

Thank you @ctrombley and @Maed223