fix: avoid unnecessary stringify in server attributes (#18232)

### Before submitting the PR, please make sure you do the following

- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. Fixes #10031.
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### What this changes

Server attribute template generation currently wraps each dynamic
expression in `$.stringify`, even when the compiler can prove the
expression is a string or a known constant. This reuses the existing
scope evaluation metadata so server output can avoid `$.stringify` for
proven string/constant chunks while keeping it for possibly nullish
unknown values.

The updated snapshot covers a mixed attribute with a known string,
mutable state, `null`, numeric/undefined constants, a known
string-producing `typeof`, and an unknown prop value.

### Tests and linting

- [x] `pnpm test snapshot -t nullish-coallescence-omittance`
- [x] `pnpm test snapshot`
- [x] `pnpm --filter svelte check`
- [x] `pnpm lint`
- [x] `pnpm prettier --check .changeset/slow-bikes-serve.md`

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Co-authored-by: Rich Harris <hello@rich-harris.dev>