avoid ignoring missing stack error

[jordanstephens]

Description

I tried to run

defang cd cancel --project-name=html-css-js --stack=smoketestaws --workspace=DefangLabs

And got back an error that said

the CD command is not valid for the Defang playground; did you forget --stack or --provider?

Looking more closely, it looks like we swallowed the missing stack error. This PR restores it.

Linked Issues

Checklist

• I have performed a self-review of my code
• I have added appropriate tests
• I have updated the Defang CLI docs and/or README to reflect my changes, if necessary

Summary by CodeRabbit

• Bug Fixes

  • Clearer, specific error when a specified stack does not exist and improved fallback messaging when no default stack is found.
  • Loading now returns immediately for explicitly missing stacks; provider fallback behavior adjusted to ensure a consistent provider value in failure paths.

• Tests

  • Added and updated tests to cover specified non-existent stack scenarios and the updated missing-stack error messaging.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a concrete stacks.ErrNotExist error returned when a requested stack is missing; session stack-loading now detects that error via errors.As and returns it immediately for specified stacks. Tests updated to expect the new error form and mock behavior adjusted accordingly.

Changes

Cohort / File(s)	Summary
Stacks manager src/pkg/stacks/manager.go	Adds public ErrNotExist type (StackName string) and Error() method; surfaces ErrNotExist when a specified/remote stack is missing and adjusts fallback messaging.
Stacks tests src/pkg/stacks/manager_test.go	Updates test expectations to the new specific missing-stack error message (e.g., stack "missingstack" does not exist).
Session loader src/pkg/session/session.go	Imports errors; SessionLoader.loadStack uses errors.As to detect *stacks.ErrNotExist for specified missing stacks and returns it immediately; preserves other error/fallback paths and ProviderAuto -> ProviderDefang adjustment.
Session tests src/pkg/session/session_test.go	Adds "specified non-existing stack" test case; mocks GetStack to return stacks.ErrNotExist when a non-empty stack is specified, and a generic error when no stack specified.

Sequence Diagram(s)

(omitted — changes are localized to error type and handler and do not introduce a multi-component sequential flow requiring visualization)

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• Lio/fix default stack #1820: Related stack-retrieval and fallback changes that touch stack-not-found handling and session↔stacks-manager interactions.

Suggested reviewers

• jordanstephens

Poem

🐇 I hopped through code and sniffed the nest,
A missing stack put me to test.
Now ErrNotExist chirps clear and bright,
No blind guesses in the night —
I twitch my nose; the fix feels right. 🥕

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: introducing specific error handling for missing stacks instead of silently ignoring them.

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✨ Finishing touches

• 📝 Generate docstrings

Warning

There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 golangci-lint (2.5.0)

level=warning msg="[linters_context] running gomodguard failed: unable to read module file go.mod: current working directory must have a go.mod file: if you are not using go modules it is suggested to disable this linter"
level=error msg="[linters_context] typechecking error: pattern ./...: directory prefix . does not contain main module or its selected dependencies"

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@src/pkg/stacks/manager.go`:
- Around line 228-231: Do not blanket-convert all errors from sm.GetRemote into
ErrNotExist; instead, call stack, err = sm.GetRemote(ctx, name) and if err !=
nil inspect it (using errors.Is/As against the remote-not-found sentinel/type
returned by sm.GetRemote or by checking a dedicated IsNotFound helper) and only
return &ErrNotExist{StackName: name} when the remote error explicitly indicates
the stack is missing; otherwise propagate or wrap the original err (e.g., return
nil, "", fmt.Errorf("GetRemote %s: %w", name, err)) so network/auth/fabric
errors are preserved.