## Summary
Removes an unsupported negative regex lookbehind from input-helper
entity ID validation.
Advanced Camera Card v8 fails to load on Safari/iPadOS 15 because the
browser cannot parse `(?<!_)`. This prevents the `advanced-camera-card`
custom element from registering.
The replacement preserves the existing validation behavior without
lookbehind:
- Accepts valid `input_*` helper entity IDs.
- Rejects object IDs beginning or ending with `_`.
- Rejects object IDs containing `__`.
- Restores compatibility with Safari versions before 16.4.
This closes#2753
## Testing
- Added regression coverage for valid and invalid input-helper entity
IDs.
- TypeScript typecheck passes.
- ESLint passes for the changed files.
- Build succeeds.
- Confirmed the generated distribution contains no negative lookbehind
syntax.
- Direct validation of the replacement regex passes.
## Reproduction
- Device: iPad mini 4
- OS: iPadOS 15.8.8
- Last working release: v7.27.4
- Affected releases: v8.0.0 and v8.0.1
- Browser error:
```text
SyntaxError: Invalid regular expression: invalid group specifier name
Safari and Safari on iOS did not support regex lookbehind until version 16.4.
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Co-authored-by: dermotduffy <dermot.duffy@gmail.com>
