feat(json-rpc-engine): Permit passing middleware functions to compatibility adapters

[rekmarks]

Explanation

As a convenience, permit passing one or more middleware functions or an engine to asV2Middleware and asLegacyMiddleware.

References

N/A

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

Enable asV2Middleware and asLegacyMiddleware to accept one or multiple middleware functions (in addition to engines), with docs, tests, and changelog updates.

• API (compatibility adapters)
  • asV2Middleware(...): add overloads to accept single/multiple legacy JsonRpcEngine middlewares; composes via mergeMiddleware; existing engine support unchanged.
  • asLegacyMiddleware(...): add overloads to accept single/multiple V2 JsonRpcMiddleware; wraps them in a temporary V2 engine internally.
• Behavior & Tests
  • Verify composing multiple middlewares, forwarding results/errors, and continuing pipeline when middleware doesn’t end.
  • Confirm context/request propagation between legacy and V2 paths.
• Docs
  • Update README.md and src/README.md with new usage examples (engine vs. middleware conversion) and context propagation notes.
• Changelog
  • Append PR reference to JsonRpcEngineV2 entry in CHANGELOG.md.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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[rekmarks]

@cursor lint the package from root via yarn eslint --fix packages/json-rpc-engine, make changes, and repeat until linting passes.