Add exhaustive test coverage for the RFC 3339 duration format

Adds per-variant cases to rfc3339_duration_test.cc that the existing tests
did not cover: every date x time shape, week exclusivity (including the
zero-valued-component case), leading and internal sign, fraction and comma,
exponent, a dangling number before T, component order and repeats, T-edge
cases, boundary values, and BOM/NUL/control bytes. All new cases pass
against the current is_rfc3339_duration.

Author: vtushar06