spawn resume: region-from-defaults type assertion panics if params file has no region default

[scttfrdmn]

Context

Found while implementing #494 (spawn resume --max-concurrent-auto), which needs a reliably-set region — this bug sits directly upstream of that.

cmd/resume.go's region-resolution logic:

region := ""
for _, inst := range state.Instances {
    if inst.InstanceID != "" {
        region = paramFormat.Defaults["region"].(string)
        break
    }
}
if region == "" {
    region = "us-east-1" // Default
}

The bug

paramFormat.Defaults["region"].(string) is an unchecked (non-, ok) type assertion. If the sweep's original parameter file has no top-level region in defaults (e.g. every param set set its own region individually, or the sweep only ever used the CLI's --region flag rather than a param-file default), Defaults["region"] is nil, and nil.(string) panics rather than falling through to the region == "" → "us-east-1" fallback below it.

The panic is reachable only when state.Instances has at least one entry with a non-empty InstanceID (i.e. at least one instance was actually launched before the sweep was interrupted) AND the param file's defaults map has no region key or a non-string one — otherwise the loop body never runs and the region == "" fallback below correctly applies.

Ask

Use the checked form (region, _ = paramFormat.Defaults["region"].(string)), or better, derive region the same way the rest of the per-config logic already does (buildLaunchConfigFromParams's per-entry region, falling back through defaults) rather than reaching into Defaults directly and assuming a top-level scalar. Worth checking whether state.Instances[i]'s own recorded config/tags could just carry the region it actually used, removing the guesswork entirely.

Not filed as blocking anything — spawn resume's existing --max-concurrent path has lived with this since before #492/#494; flagging it now because #494's --max-concurrent-auto makes correctness of region matter more (a wrong/panicked region means a wrong quota answer, not just a wrong ListInstances region filter).