pulumi module shows no stack on Pulumi v3.256.0 and later

Bug Report

Current Behavior

The pulumi module renders an empty $stack on Pulumi v3.256.0 and later.

Expected Behavior

$stack shows the selected stack, as it does on Pulumi v3.255.0 and earlier.

Additional context/Screenshots

pulumi/pulumi#23974 ("fix(cli): scope current stack selection to the active backend", first released in v3.256.0) changed the workspace settings file so that the selected stack is scoped to the backend it belongs to:

// ~/.pulumi/workspaces/<project>-<sha1>-workspace.json, before
{
    "stack": "acme/infra/dev"
}

// after
{
    "stacks": {
        "https://api.pulumi.com": "acme/infra/dev"
    }
}

The old stack key is still read by Pulumi for backwards compatibility, but it is no longer written — from the comment on Settings in pkg/workspace/settings.go:

Deprecated: Prefer Stacks[backendURL]. Kept so older workspace.json files continue to load; new writes use Stacks.

So the prompt keeps working after the upgrade until the first pulumi stack select, and then goes quiet.

src/modules/pulumi.rs only knows about the old key:

workspace
    .as_object()?
    .get("stack")?
    .as_str()
    .map(ToString::to_string)

Possible Solution

Read stacks[<backend url>], resolving the active backend the way the CLI does (PULUMI_BACKEND_URL, then the project's backend.url, then current in credentials.json), and keep the old stack key as a fallback for workspace files that have not been rewritten yet. Happy to open a PR.

Environment

• Starship version: 1.26.0
• Shell type: zsh
• Shell version: 5.9
• Shell plugin manager: none
• Terminal emulator: n/a — the module is shell and terminal independent
• Operating system: macOS 26.5.2
• Pulumi version: 3.256.0+

Relevant Shell Configuration

eval "$(starship init zsh)"

Starship Configuration

[pulumi]
format = '[$symbol$stack\(@$username\)]($style) '
style = 'bold fg:#F4B321'