Tf 34724 terraform stacks cli should be easier to use with non standard hosts tfe

[lakshyaa-nathan-15]

Summary

Improves hostname resolution for the Stacks plugin by automatically inferring the target hostname from the local credentials file (credentials.tfrc.json) when no explicit hostname environment variable is set.

Previously, the StacksCommand only checked the TF_STACKS_HOSTNAME environment variable and fell back to the default hostname (app.terraform.io) if it was unset. This meant users running Terraform Enterprise (TFE) instances needed to always remember to set the environment variable explicitly.

Changes

The new resolution logic follows this priority order:

1. TF_STACKS_HOSTNAME - used directly if set.

2. TF_CLOUD_HOSTNAME - used if set and TF_STACKS_HOSTNAME is not

3. Check for credentials file override

   a. Single entry in credentials.tfrc.json - if exactly one hostname is present in the credentials file, it is used automatically with a warning message

   b. Single entry in custom credentials file - if exactly one hostname is present in the credentials file, it is used automatically with a warning message

4. Multiple entries in credentials file - an error diagnostic is returned because the intended host is ambiguous; the user must set TF_STACKS_HOSTNAME or TF_CLOUD_HOSTNAME to disambiguate.

5. Default hostname - falls back to app.terraform.io when no credentials are present.

Additional changes:

• Added ReadCredentialHostsInOrder (and internal readCredentialHostsInOrderFile) to internal/command/cliconfig. This function parses the credentials file using a JSON decoder so that the original declaration order of hostnames is preserved
• Improved the error message for hostname discovery failure to include a hint about setting TF_STACKS_HOSTNAME or TF_CLOUD_HOSTNAME.

Testing

Added/updated tests covering:

• TF_STACKS_HOSTNAME set explicitly
• TF_CLOUD_HOSTNAME fallback
• Credentials file override
• Single credential entry auto-resolution with warning
• Multiple credential entries returning ambiguity error
• Default hostname fallback when no credentials are present
• ReadCredentialHostsInOrder preserving declaration order

Target Release

1.16.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[hashicorp-cla-app]

All committers have signed the CLA.

[jondavidjohn]

I actually don't think we need the warning here, only when there are more than 1. I think we can think of a single entry as the happy path 👍