fix(models): avoid panic when matching trigger items with regex conditions

[devteamaegis]

What's broken

bitrise trigger-check (and trigger matching generally) panics with interface conversion: interface {} is map[string]interface {}, not string whenever a trigger_map item uses a regex condition, e.g.:

trigger_map:
- push_branch: { regex: "feature-.*" }
  workflow: primary

Why it happens

stringFromTriggerCondition did an unchecked value.(string). Regex conditions are stored as a map ({regex: ...}), not a string, so MatchWithParams -> FirstMatchingTarget panics. The validator already accepts regex-map conditions, so this config is otherwise valid.

Fix

Delegate stringFromTriggerCondition to the existing stringLiteralOrRegex, which already handles nil, plain strings, and the regex-map shapes — returning the regex string instead of panicking. Behavior is unchanged for existing string/nil inputs.

Test

Added TestTriggerMapItemModel_MatchWithParams_RegexCondition, which panics before the change and passes after. Full models package test suite stays green.