Fix infinite loop when INTERVAL=0 in recurrence rules

[anushka-codes1]

Fixes #39

Problem

Setting an interval ("repeat every") value of "0" in the recurrence popup causes an infinite loop in Python dateutil RRULE interpretation, hanging the server.

Root Cause

• INTERVAL=0 is not a valid value per RFC 5545
• python-dateutil enters an infinite loop when iterating over such rules
• The frontend mockup has validation for this, but server-side validation was missing

Solution

Added server-side validation in json_recurrence.py to:

• Detect RRULE strings containing INTERVAL=0 or negative values using regex
• Return HTTP 400 with the existing translated error message no_repeat_every
• Prevent the infinite loop by rejecting invalid intervals before calling rrule.rrulestr()

Testing

• Syntax check passed with python -m py_compile
• Manual testing: POST to @@json_recurrence with INTERVAL=0 now returns 400 + error message instead of hanging

Example

Before: FREQ=MONTHLY;INTERVAL=0;BYMONTHDAY=1;COUNT=7 → infinite loop
After: Same RRULE → HTTP 400 with translated error message

Files Changed

• src/plone/formwidget/recurrence/browser/json_recurrence.py - Added validation logic
• news/interval-validation.bugfix - Towncrier changelog entry

[mister-roboto]

@anushka-codes1 thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

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Happy hacking!

[anushka-codes1]

@jenkins-plone-org please run jobs