Add Node.js 26 support

[Poggen]

Description

auth0@5.12.0 currently declares support for Node.js ^20.19.0 || ^22.12.0 || ^24.0.0. Node.js 26 is now available, but consumers running with strict engine checks cannot install the package even though the SDK appears to run correctly on Node 26.

Current behavior

With node:26-alpine:

npm install auth0@5.12.0 --engine-strict --ignore-scripts

fails with:

npm error code EBADENGINE
npm error notsup Required: {"node":"^20.19.0 || ^22.12.0 || ^24.0.0"}
npm error notsup Actual:   {"node":"v26.3.0","npm":"11.16.0"}

Without strict engines, installation only warns and basic ESM imports/constructing ManagementClient work.

Validation

Running the upstream validation suite under Node.js v26.3.0 with the current engine guard bypassed passes:

• 203 test suites passed
• 4,464 tests passed
• yarn lint:package passed, with the existing publint suggestion only

Proposed change

Add Node.js 26 to the supported engine range, README/runtime compatibility docs, and CI test matrix.