## Summary
- Fixes the property parser regex to allow `+` in key names (e.g.
`abc+def=etc`)
- The regex from #8387 used `[^=+]+` for key matching, which excluded
`+` from key names entirely
- Changed to a lazy `.+?` match so `+` is only treated as the `+=`
operator when immediately before `=`
This is a small targeted fix and does not address all edge cases with
the `+=` syntax.
## Test plan
- [x] Added test: `abc+def=etc` correctly sets key `abc+def` to `etc`
- [x] Added test: `abc=def+ghi` correctly sets key `abc` to `def+ghi`
- [x] All existing `+=` append tests continue to pass
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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