fix(spur-cli): reject unresolved usernames

[01xjw]

Summary

• Replace username fallback sentinels with a fail-closed Result from current_user().
• Propagate lookup failures through submission, control, exec, attach, and output-streaming paths.
• Reuse the owner stored in each submitted JobSpec for later cancellation instead of looking it up again.

Closes #567.

Validation

• cargo test -p spur-cli --locked
• cargo fmt --all --check
• cargo clippy -p spur-cli --all-targets --locked -- -D warnings
• Runtime check under an unmapped UID exits non-zero with failed to determine current username and never emits a sentinel.
• Deterministic baseline failed before the change and passed afterward.

Disclosure

This change was prepared with assistance from the radeon-issue automation and independently checked by a separately configured validation model. Maintainer review is still required.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 17.50000% with 33 lines in your changes missing coverage. Please review.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #569   +/-   ##
=======================================
  Coverage   76.03%   76.03%           
=======================================
  Files         166      166           
  Lines       63002    62988   -14     
=======================================
- Hits        47898    47890    -8     
+ Misses      15104    15098    -6     

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[yansun1996]

LGTM — clean fail-closed fix.

[yansun1996]

Approving. One optional follow-up worth tracking separately: k8s.rs effective_user() (line 112) still uses the old "unknown" sentinel and feeds caller into the k0s/kubeconfig RPCs — the last one left in spur-cli. Not a bypass (is_k0s_admin rejects "unknown"), so fine outside this PR.

[Copilot]

Pull request overview

This PR hardens spur-cli’s authenticated job operations by making local username resolution fail closed, preventing jobs from being recorded or accessed under inconsistent “fallback” owner values when the OS cannot resolve the current UID.

Changes:

• Change interactive::current_user() to return Result<String> and propagate failures through multiple CLI request paths.
• Remove "unknown" fallback usernames in several commands (submit/control/exec/attach/output streaming), replacing them with error propagation.
• Reuse the JobSpec.user captured at submission time for later cancellation in srun/salloc to avoid re-looking up the username.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/spur-cli/src/interactive.rs	Make username resolution fail closed (Result) and map lookup failures into RPC status where needed.
crates/spur-cli/src/srun.rs	Propagate current_user()? into job spec/step/output streaming; reuse submitted owner for cancellations; adjust output streaming to return Result<bool>.
crates/spur-cli/src/salloc.rs	Use current_user()? in job spec and reuse submitted owner for cancellation paths.
crates/spur-cli/src/sbatch.rs	Use current_user()? instead of a fallback username when building the submitted JobSpec.
crates/spur-cli/src/scancel.rs	Use current_user()? when --user is not provided instead of falling back to "unknown".
crates/spur-cli/src/scontrol.rs	Use current_user()? for authenticated control-plane requests (cancel/suspend/resume/reservation updates).
crates/spur-cli/src/sattach.rs	Use current_user()? for output streaming authentication.
crates/spur-cli/src/exec.rs	Use current_user()? for authenticated exec requests.
crates/spur-cli/src/scrontab.rs	Make the effective user lookup fail closed by returning Result<String> and propagating errors into user-facing messages.

Suppressed comments (1)

crates/spur-cli/src/scrontab.rs:74

• list_crontab calls effective_user()? separately in each branch, which repeats the OS username lookup and can produce inconsistent output if resolution changes. Resolve once at the top and reuse the string for all messages.

fn list_crontab(path: &PathBuf) -> Result<()> {
    if path.exists() {
        let content = std::fs::read_to_string(path)?;
        if content.trim().is_empty()
            || content

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