fix(config): correctly validate cacheHandlers names (#95358)
## What?
The current `cacheHandlers` validation uses `/[a-z-]/`, which only
checks whether a key contains at least one lowercase letter or hyphen.
As a result, invalid keys such as `abc123`, `abc_123`, `abc.def`, and
`handler!` are accepted even though they contain characters outside the
documented allowed set.
## Why?
The validation error states that handler names must only contain `a-z`
and `-`, but the current implementation does not enforce that constraint
because the regex is not anchored.
## How?
- Replace `/[a-z-]/` with `/^[a-z-]+$/` so the entire handler name is
validated.
- Add regression tests covering both valid and invalid handler names.
## Testing
- Added regression tests in `packages/next/src/server/config.test.ts`.
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