Validate container system logs --last flag (#1561)

- Closes #1530.

## Type of Change
- [x] Bug fix
- [ ] New feature  
- [ ] Breaking change
- [ ] Documentation update

## Motivation and Context
Reject invalid `--last` values in system logs, matching the format
accepted by `log show`.

## Testing
- [x] Tested locally
- [x] Added/updated tests
- [x] Added/updated docs

Tests/CLITests/Subcommands/System/TestCLISystemLogs.swift

@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import Testing
+
+@Suite(.serialSuites)
+final class TestCLISystemLogs: CLITest {
+
+    @Test func testLogsRejectsInvalidLastUnit() throws {
+        let (_, _, error, status) = try run(arguments: ["system", "logs", "--last", "1x"])
+        #expect(status != 0, "Expected non-zero exit for invalid --last unit")
+        #expect(error.contains("invalid --last value"))
+    }
+
+    @Test func testLogsRejectsNonNumericLast() throws {
+        let (_, _, error, status) = try run(arguments: ["system", "logs", "--last", "abc"])
+        #expect(status != 0, "Expected non-zero exit for non-numeric --last")
+        #expect(error.contains("invalid --last value"))
+    }
+
+    @Test func testLogsRejectsZeroLast() throws {
+        let (_, _, error, status) = try run(arguments: ["system", "logs", "--last", "0m"])
+        #expect(status != 0, "Expected non-zero exit for zero --last value")
+        #expect(error.contains("invalid --last value"))
+    }
+}