slog-handler-guide: make time regex timezone-aware

[cu-min]

The indenthandler{1..4} tests assert the log timestamp with the regex time: [-0-9T:.]+Z?, which only accepts a UTC Z suffix. On hosts with a non-UTC timezone (e.g. Asia/Shanghai, UTC+8), log/slog emits a numeric offset and the tests fail:

--- FAIL: Test (0.00s)
    indent_handler_test.go:32:
        got:
        "time: 2026-08-12T16:00:00.858525+08:00\nlevel: INFO\nsource: \".../indent_handler_test.go:16\"\nmsg: \"hello\"\na: 1\nb: true\n..."
        want:
        "time: [-0-9T:.]+Z?\nlevel: INFO\n..."

The + sign and the +08:00 offset are not covered by the [-0-9T:.] character class. This change extends the regex to also accept a numeric UTC offset:

time: [-0-9T:.]+(Z|[+-][0-9]{2}:[0-9]{2})?

Verified on a UTC+8 host: go test ./slog-handler-guide/... passes for all four handlers (previously indenthandler1 and indenthandler2 failed).

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