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 more middleware functions (in addition to engines), with docs, tests, and changelog updates.

• API:
  • asV2Middleware() now accepts a legacy JsonRpcEngine or one/more legacy middleware via rest params; composes multiple via mergeMiddleware and preserves error/result forwarding and context propagation.
  • asLegacyMiddleware() now accepts a JsonRpcEngineV2 or one/more V2 middleware via rest params; internally builds a V2 engine when given middleware and preserves request/context propagation.
• Docs:
  • Update README.md and src/README.md to document converting engines and middleware for both directions, with context propagation notes and examples.
• Tests:
  • Add/expand tests for single/multiple middleware inputs, error forwarding, and continuation behavior in asV2Middleware.test.ts and v2/asLegacyMiddleware.test.ts.
• Changelog:
  • Add PR link #6991 to JsonRpcEngineV2 entry in CHANGELOG.md.

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[cursor]

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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.