format: add RFC 3339 unknown local offset (-00:00) test for time

[Vaibhav701161]

Description

This PR adds a small but important test case for the time format covering the unknown local offset representation defined in RFC 3339.

Background

RFC 3339 (Section 4.3) explicitly defines the offset -00:00 as a valid representation indicating that the local offset is unknown. This is distinct from Z or +00:00, which represent a known UTC offset.

While this distinction is subtle, it is part of the standard and appears in real-world data where the exact offset is not available but the time is still meaningful.

Current Situation

The existing test suite includes:

• valid positive and negative offsets (e.g., +05:30, -08:00)
• invalid offset formats and malformed values

However, it does not currently include an explicit test for the -00:00 case.

During local validation with multiple implementations (Hyperjump, python-jsonschema, jschon), I observed that:

• some implementations accept this value as valid
• others reject it

This suggests that the behavior is not consistently handled across implementations, even though the RFC defines it clearly.

What This PR Adds

A single test case across relevant drafts:

{
  "description": "time with unknown local offset (-00:00)",
  "comment": "RFC 3339 section 4.3",
  "data": "12:34:56-00:00",
  "valid": true
}

Why This Matters

• It improves interoperability clarity for implementations that choose to enforce format.
• It documents a valid but currently underrepresented edge case from the RFC.
• It helps reduce ambiguity for implementers relying on the test suite as a reference.

[jdesrosiers]

Looks right to me