Detect middleware runtime and conditionally handle Node middleware for Cloudflare build

[314systems]

Motivation

• Allow newer Next.js builds (15.5+) that ship Node-style middleware/proxy to be detected and not fail the Cloudflare build.
• Avoid unconditional removal of Node middleware and instead make patching conditional based on the detected Next.js output and version.

Description

• Replaced useNodeMiddleware with detectMiddlewareRuntime which returns one of "edge", "node", or "none" by inspecting compiled Next output manifests.
• Updated build.ts to call detectMiddlewareRuntime and to log an informational message when Node middleware is detected instead of erroring and exiting.
• Adjusted next-server AST patching so the disableNodeMiddlewareRule is applied only for Next.js versions less than 15.5.0 using compareSemver.
• Added middleware.spec.ts unit tests for detectMiddlewareRuntime and updated related imports and comments explaining why internal patches remain in place.

Testing

• Ran unit tests with vitest, including the new packages/cloudflare/src/cli/build/utils/middleware.spec.ts, and all tests passed.
• Existing patch/unit specs that exercise next-server patches were executed and succeeded under the test suite.

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Codex Task