fix: allow $schema key in serve.json

[charan-rathore]

Related Issues

• Fixes Allow JSON schema property in serve.json #862

Description

Changed

• Strip the top-level $schema key from serve.json before validation, so editor-added schema references no longer fail with "must NOT have additional properties". It is editor metadata, not a serve option.
• Added tests/config-schema-key.test.ts (a config with $schema plus real options still loads the options; a $schema-only config is accepted) and a patch changeset.

Caveats/Problems/Issues

New tests fail 2 of 3 on the unpatched base and pass 3 of 3 with the change. With a serve.json containing $schema and cleanUrls: false, the compiled CLI serves HTTP 200; the unpatched build exits with the AJV error.

Checklist

• The issues that this PR fixes/closes have been mentioned above.
• What this PR adds/changes/removes has been explained.
• All tests (pnpm test) pass.
• The linter (pnpm lint) does not throw an errors.
• All added/modified code has been commented, and
  methods/classes/constants/types have been annotated with TSDoc comments.

[vercel]

@charan-rathore is attempting to deploy a commit to the Internal Playground Team on Vercel.

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[charan-rathore]

Closing this since there has been no movement on it. Happy to reopen and pick it back up if there is interest.