## Description
Follow-up to #15319, addressing the non-blocking review notes and the
Ito QA finding there.
The `days_before` filter value went straight through `to_i`, which never
fails: `-1` moves the cutoff into the future and matches every
conversation, and a non-numeric string becomes `0`. The value is now
parsed as a base-10 integer and must fall within the UI-supported
`1..998` range; anything else raises the existing
`CustomExceptions::CustomFilter::InvalidValue`, which the controller
already turns into a client error.
Also corrects an existing weak spec that sent `3` days but computed its
expectation with `2` days, passing only because the seeded data made
both counts equal. It now sends `2` and exercises the exclusive
boundary.
Refs https://linear.app/chatwoot/issue/CW-7832
## Type of change
- [x] Bug fix (non-breaking change which fixes an issue)
## How Has This Been Tested?
- New specs: invalid values (`-1`, `abc`, `0`, `999`) raise
`InvalidValue`; string values parse as base 10 (`'02'` means 2 days, not
octal).
- `bundle exec rspec spec/services/conversations/filter_service_spec.rb`
(36 examples, 0 failures).
## Checklist:
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules
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