feat: set TF_APPEND_USER_AGENT env var for terraform steps

[aidansteele]

Note: this was originally implemented in #5588. I tried to implement this with a merge commit, plus my standalone fixes, plus addressing the feedback from @nitrocode (on the original PR), but I couldn't get it to apply cleanly. So this PR implements the same functionality squashed into a single commit, with the original PR's author added as a co-author.

what

Set TF_APPEND_USER_AGENT before all terraform step executions so that providers include Atlantis context in their User-Agent headers. This makes it easier to correlate actions in provider audit logs (e.g. AWS CloudTrail) with the PR/commit that triggered them.

The value uses the --vcs-status-name (defaults to 'atlantis') as the product token, allowing operators running multiple Atlantis instances to differentiate between them.

Format: <vcs-status-name>/<version> (<user>; <command>; <dir>; <workspace>; <commit>; +<pr-url>)

why

It's useful to have this information in logs (ex: CloudTrail) to identify exactly which PR modified a resource.

tests

Updated tests to pass with these changes

references

• feat: set TF_APPEND_USER_AGENT environment variable #5588
• closes Docs on appending the user agent with relevant atlantis information #2651

[Copilot]

Pull request overview

This PR adds a default TF_APPEND_USER_AGENT environment variable to Atlantis’ Terraform-related step executions so downstream Terraform providers can include PR/commit context in their HTTP User-Agent, improving audit-log correlation across runs (e.g., CloudTrail).

Changes:

• Populate TF_APPEND_USER_AGENT in DefaultProjectCommandRunner.runSteps() for all workflow steps, using --vcs-status-name (or atlantis) and the Atlantis version plus PR context.
• Wire AtlantisVersion and VCSStatusName into DefaultProjectCommandRunner from server initialization.
• Update unit tests and docs to account for the new environment variable behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
server/server.go	Passes Atlantis version and VCS status name into the project command runner so it can compose the user agent.
server/events/project_command_runner.go	Builds and injects TF_APPEND_USER_AGENT into the env map before executing workflow steps.
server/events/project_command_runner_test.go	Updates expected env maps for step runner calls to include TF_APPEND_USER_AGENT.
server/core/runtime/run_step_runner_test.go	Ensures custom run steps receive and can reference TF_APPEND_USER_AGENT.
runatlantis.io/docs/custom-workflows.md	Documents TF_APPEND_USER_AGENT as a native environment variable and provides an example format.

[Copilot]

The new TF_APPEND_USER_AGENT composition logic (version truncation + VCS status name fallback) isn’t directly covered by unit tests with non-empty values. Current expectations in project_command_runner_test mostly exercise the zero-value defaults, so a regression in how AtlantisVersion/VCSStatusName are used (or truncated) could slip through. Consider adding/adjusting a test case that sets runner.AtlantisVersion (including a verbose value with spaces) and runner.VCSStatusName, and asserts the resulting TF_APPEND_USER_AGENT matches the documented format.

[nitrocode]

cc @aidansteele

[aidansteele]

@nitrocode I realised the PR was no longer merging cleanly, and the DCO check was failing - I've fixed both. Let me know if there's anything else I can do.