fix(api): populate volume mounts and normalize alias for paused sandboxes in GetSandboxesSandboxID

[chill-czar]

Closes #3575

Summary

• Populate VolumeMounts from lastSnapshot.Snapshot.Config.VolumeMounts in GetSandboxesSandboxID for paused sandboxes using convertFromDBMountsToAPIMounts, matching GET /v2/sandboxes.
• Normalize sandbox.Alias so it remains nil (omitted from JSON) when len(lastSnapshot.Aliases) == 0 instead of pointing to an empty string &"".
• Add defensive nil guard for a.orchestrator != nil before querying active sandboxes.
• Add test coverage in packages/api/internal/handlers/sandbox_get_test.go (TestGetSandboxesSandboxID_PausedWithVolumeMountsAndNilAlias).

Why

In packages/api/internal/handlers/sandbox_get.go, GET /sandboxes/{id} did not populate VolumeMounts for paused sandboxes, causing a discrepancy with GET /v2/sandboxes where snapshot volume mounts are properly returned. Furthermore, firstAlias returns "" when there are no aliases; assigning a pointer to this string caused JSON output to serialize "alias": "" instead of omitting the field or returning null.

Diff Overview

 		var alias *string
 		if len(lastSnapshot.Aliases) > 0 {
 			first := lastSnapshot.Aliases[0]
 			alias = &first
 		}
 
+		var volumeMounts []queries.VolumeMount
+		if lastSnapshot.Snapshot.Config.VolumeMounts != nil {
+			volumeMounts = *lastSnapshot.Snapshot.Config.VolumeMounts
+		}
+
 		sandbox = api.SandboxDetail{
 			ClientID:     lastSnapshot.Snapshot.BaseTemplateID,
 			TemplateID:   lastSnapshot.Snapshot.BaseTemplateID,
 			Alias:        alias,
+			VolumeMounts: convertFromDBMountsToAPIMounts(volumeMounts),
 			SandboxID:    sandboxID,

Test Plan

• Unit test: go test -v ./packages/api/internal/handlers -run TestGetSandboxesSandboxID_PausedWithVolumeMountsAndNilAlias
• Package test suite passes: go test -v ./packages/api/internal/handlers/...
• Linter & Formatter pass: make fmt and make lint
• Verified VolumeMounts is preserved on paused sandboxes and Alias is omitted when empty

/cc @jakubno @dobrac @ValentaTomas @arkamar @tvi

[chill-czar]

@claude review