[rubensdemelo]

Motivation

• Provide a convenient extension that formats date and time including seconds in the common Brazilian pattern dd/MM/yyyy HH:mm:ss so callers don't need to combine formats manually.
• Ensure this new use-case is covered by docs, example and tests to prevent regressions for padded and non-padded time values.

Description

• Add diaMesAnoHoraMinutoSegundo() to lib/brasil_datetime.dart, which returns dd/MM/yyyy HH:mm:ss and preserves the package's lazy pt_BR initialization via _garantirInicializacao().
• Add unit tests exercising the new formatter and padded/non-padded time cases in test/brasil_datetime_test.dart.
• Document the new method in README.md and demonstrate it in example/brasil_datetime_example.dart.
• Record the change in CHANGELOG.md under the Unreleased section.

Testing

• Ran dart format on modified files with no formatting issues detected.
• Ran dart analyze with no analyzer issues reported.
• Ran dart test and all tests passed (72 tests in total).
• Ran dart run example/brasil_datetime_example.dart and the example produced the expected date/time lines, and git diff --check reported no issues.

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Codex Task