feat: Tests for product module channel input

[MichaelThamm]

• Merge this into main as well
• Update the READMEs so they mention track/2
• Fix the just lint-terraform-docs since it doesn't actually lint, it just formats it.
  • Add this to the central TF CI (requires each charm repo to have a tf-docs file?)

Issue

If a user specifies the TF COS (or COS Lite) modules

1. without a ref "git::.../observability-stack//terraform/cos-lite then they must use track dev
2. with a ref "git::.../observability-stack//terraform/cos-lite?ref=track/2 then they must use track 2

I noticed some users make this mistake (and I) where you try out a different track, e.g. dev, but leave the module source as ?ref=track/2 and notice a failure to deploy.

Solution

module "cos-lite" {
  source                          = "../../cos-lite"
  model_uuid                      = juju_model.cos-lite.uuid
  channel                         = "3/stable"
}

╷
│ Error: Invalid value for variable
│ 
│   on root.tf line 19, in module "cos-lite":
│   19:   channel                         = "3/stable"
│     ├────────────────
│     │ var.channel is "3/stable"
│ 
│ The track (the part before the '/') of the channel must match the default.
│ 
│ This was checked by the validation rule at ../../cos-lite/variables.tf:18,3-13.

Checklist

• I have added or updated relevant documentation.
• PR title makes an appropriate release note and follows conventional commits syntax.
• Merge target is the correct branch, and relevant tandem backport PRs opened.

Context

Testing Instructions

Running Terraform tests:

• tf -chdir=terraform/cos test
• tf -chdir=terraform/cos-lite test

will result in running unit tests for channel validation

tests/channel_validation.tftest.hcl... in progress
  run "valid_channel_track"... pass
  run "valid_channel_stable"... pass
  run "valid_channel_candidate"... pass
  run "valid_channel_beta"... pass
  run "invalid_channel_track"... pass
  run "invalid_channel_track_numeric"... pass
  run "invalid_channel_track_string"... pass
  run "invalid_channel_track_dev"... pass
tests/channel_validation.tftest.hcl... tearing down
tests/channel_validation.tftest.hcl... pass

Success! 8 passed, 0 failed.

Upgrade Notes

[MichaelThamm]

Suggested change

	condition = split("/", var.channel)[0] == split("/", "2/stable")[0]
	condition = split("/", var.channel)[0] == "2"

[MichaelThamm]

or even better, use starts_with("3-lts")

[MichaelThamm]

Update this to specify the actual track, not "default"