feat(json-rpc-engine): Add v2 scaffold middleware

[rekmarks]

Explanation

Adds v2/createScaffoldMiddleware.ts to match the equivalent legacy middleware.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

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Note

Introduce v2/createScaffoldMiddleware, export it, and update tests/changelog with minor type improvements.

• v2:
  • Middleware: Add createScaffoldMiddleware to map RPC methods to handlers or primitive results.
  • Exports: Re-export createScaffoldMiddleware from v2/index.ts and update export snapshot.
  • Types: Default generics for MiddlewareParams in JsonRpcEngineV2 to improve ergonomics.
  • Tests:
    • Add createScaffoldMiddleware.test.ts.
    • Improve tests/utils.makeRequest typing; remove unnecessary @ts-ignore in parallel requests test.
• Changelog: Append PR reference #6975 to JsonRpcEngineV2 entry.

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