fix: skip schedules with invalid cron_offset instead of crashing the scheduler

[juanmicl]

Closes: #668

is_cron_task_now called ZoneInfo(offset) outside the try/except
that converts errors into CronValueError, and the scheduler loop only
catches CronValueError. One schedule with a non-IANA cron_offset
string (e.g. the natural spelling "UTC+3") raised
ZoneInfoNotFoundError out of SchedulerLoop.run and killed the whole
scheduler process.

With this change a bad offset goes through the same path as an invalid
cron expression: CronValueError is raised, the loop logs the existing
Cannot parse cron warning and skips that schedule.

Two regression tests cover invalid offset strings: the natural spelling
"UTC+3" and a typo'd timezone name ("Europa/Madrid"). Both fail on
master with ZoneInfoNotFoundError and pass with this change.

Complements #625 (which fixes duration-string offsets at
model-validation time but leaves the runtime guard unhandled).

Validation: full suite passes (pytest -q, 324 tests), black, ruff
and mypy clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.65%. Comparing base (c4b8666) to head (5fa2c05).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #669      +/-   ##
==========================================
+ Coverage   82.63%   82.65%   +0.01%     
==========================================
  Files          69       69              
  Lines        2730     2733       +3     
==========================================
+ Hits         2256     2259       +3     
  Misses        474      474              

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