Adds CreatedBy to AgentToken

[jpadrianoGo]

Description

This PR adds CreatedBy relation to AgentToken struct.

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 TestAgentTokensReadCreatedBy. The new tests should pass.
3. CreatedBy relation is read for AgentToken.

{
  "data": {
    "id": "at-***",
    "type": "authentication-tokens",
    "attributes": {
      "created-at": "2025-07-05T01:50:16.218Z",
      "last-used-at": null,
      "description": "this agent token is created by user-***",
      "token": null,
      "expired-at": null
    },
    "relationships": {
      "team": {
        "data": null
      },
      "created-by": {
        "data": {
          "id": "user-***",
          "type": "users"
        }
      }
    }
  }
}

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 TestAgentTokensReadCreatedBy

=== RUN   TestAgentTokensReadCreatedBy
--- PASS: TestAgentTokensReadCreatedBy (3.56s)
PASS
ok      github.com/hashicorp/go-tfe     3.614s

[ctrombley]

👍

[ctrombley]

Superseded by #1158.

[jpadrianoGo]

thank you @ctrombley