[BUG] Default Python datetimeFormat is not RFC3339 compliant

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

OpenAPI documentation suggests that both date and date-time formats should be RFC3339 compliant, but the default for Python is not, as it is missing a colon in the timezone offset field. (%Y-%m-%dT%H:%M:%S%z).

openapi-generator version

7.11.0

OpenAPI declaration file content or url

timestamp:
    type: string
    format: date-time

Generation Details

Steps to reproduce

Add a date-time formatted string field as a parameter, the generated python code will send timestamps in the format (%Y-%m-%dT%H:%M:%S%z) e.g. 2025-03-25T12:32:00+0900

Related issues/PRs

Suggest a fix

In Python 3.12+, changing the %z directive to %:z will add a colon to the timezone offset, making the timestamp RFC3339 compliant.

Start with the Python generator behavior described in the issue and reproduce it using the supplied OpenAPI schema with a date-time field. Trace where the default datetime format is selected; the work is done when generated timestamps include the colon required by RFC3339 and the reproduction no longer produces an offset such as +0900.