feat: JsonRpcEngineV2 (#6176)

## Explanation

Introduces `JsonRpcEngineV2` and `JsonRpcServer`, intended to replace
all existing usage of the existing `JsonRpcEngine` implementation. For
the motivation behind this change, see #6088 and/or [this
ADR](https://github.com/MetaMask/decisions/blob/main/decisions/core/0013-json-rpc-engine-rewrite.md).

### Implementation

In order to resolve the problems listed in the motivation, V2 engine is
split in two:
- `JsonRpcEngineV2`
  - Orchestrates middleware and composes sub-engines
- JSON-RPC requests go in, result values (not JSON-RPC responses) come
out
  - Re-throws middleware errors directly
- `JsonRpcServer`
  - JSON-RPC requests go in, JSON-RPC responses come out
- Accepts an `errorHandler` constructor parameter for capturing engine /
middleware errors

See the updated package `README.md` for details.

### Migration

While this PR is substantial, migrating our existing JSON-RPC pipelines
will be a significant project involving multiple teams over many
releases cycles. To facilitate this, the PR introduces a
forwards-compatibility adapter, `asV2Middleware()`, and
backwards-compatibility adapter, `asLegacyMiddleware()`, for the legacy
and V2 engines, respectively. In addition, all V2 exports are exposed
under the `/v2` export path, making this update completely non-breaking
(although all legacy exports are deprecated).

### Note to reviewers

I recommend proceeding as follows:

0. Read the readme
1. `JsonRpcEngineV2`
2. `JsonRpcServer`
3. Compatibility adapter functions

## References

- Closes #6088
- [ADR: Rewriting
`JsonRpcEngine`](https://github.com/MetaMask/decisions/blob/main/decisions/core/0013-json-rpc-engine-rewrite.md)

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
- ~I've prepared draft pull requests for clients and consumer packages
to resolve any breaking changes~
   - Migrating our RPC pipelines left as an exercise to the reader.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Introduce `JsonRpcEngineV2` and `JsonRpcServer` with bidirectional
adapters, deprecating the legacy engine, updating docs/exports/deps, and
adding comprehensive tests.
> 
> - **Core (v2)**:
> - Add `JsonRpcEngineV2` with typed middleware pipeline, immutable
request/result handling, and `MiddlewareContext`.
> - Add `JsonRpcServer` for spec-compliant request/notification handling
and error serialization.
> - Provide adapters: `asLegacyMiddleware` (v2 -> legacy) and
`asV2Middleware` (legacy -> v2).
> - **Exports & Packaging**:
> - Expose v2 under `@metamask/json-rpc-engine/v2` (ESM/CJS), add
`v2.js` shim for Browserify.
> - Update `package.json` exports and add deps (`deep-freeze-strict`,
`klona`, `@types/deep-freeze-strict`).
> - **Docs**:
> - Rewrite root `README.md` to v2 usage; add legacy docs in
`src/README.md` and mark legacy APIs as deprecated.
>   - Update `CHANGELOG.md`: Added v2; Deprecated legacy engine/types.
> - **Legacy**:
> - Annotate legacy engine/types with deprecation notes; minor internal
tweaks (stringify util).
> - **Tests**:
> - Add extensive v2, server, and adapter tests; adjust legacy tests as
needed.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
eca0e39. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: rekmarks

eslint-warning-thresholds.json

@@ -287,12 +287,6 @@
   "packages/gas-fee-controller/src/determineGasFeeCalculations.ts": {
     "jsdoc/tag-lines": 4
   },
-  "packages/json-rpc-engine/src/JsonRpcEngine.test.ts": {
-    "jest/no-conditional-in-test": 2
-  },
-  "packages/json-rpc-engine/src/JsonRpcEngine.ts": {
-    "@typescript-eslint/prefer-promise-reject-errors": 2
-  },
   "packages/json-rpc-middleware-stream/src/index.test.ts": {
     "@typescript-eslint/prefer-promise-reject-errors": 3,
     "no-empty-function": 1

packages/json-rpc-engine/CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `JsonRpcEngineV2` ([#6176](https://github.com/MetaMask/core/pull/6176))
+  - This is a complete rewrite of `JsonRpcEngine`, intended to replace the original implementation.
+    See the readme for details.
+
 ## [10.1.1]
 
 ### Changed
@@ -20,6 +26,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Bump `@metamask/utils` from `^11.2.0` to `^11.4.2` ([#6054](https://github.com/MetaMask/core/pull/6054))
 - Bump `@metamask/utils` from `^11.4.2` to `^11.8.0` ([#6588](https://github.com/MetaMask/core/pull/6588))
 
+### Deprecated
+
+- `JsonRpcEngine` and related types ([#6176](https://github.com/MetaMask/core/pull/6176))
+  - To be replaced by `JsonRpcEngineV2`.
+
 ## [10.0.3]
 
 ### Changed