[umeraamir69]

Summary

• Buffer.alloc(size, fill, encoding) now matches Node.js: encoding is only used/validated when fill is a string.
• Non-string fills (number/Buffer) with an encoding argument no longer throw ERR_INVALID_ARG_TYPE.
• Adds regression coverage in buffer-nodejs-test.js for the cases from node:buffer: Buffer.alloc(size, fill, encoding) throws when fill is a number or a Buffer #8286.

Closes #8286

Test plan

• bazel test //src/workerd/api/node/tests:buffer-nodejs-test
• Or reproduce with the worker from node:buffer: Buffer.alloc(size, fill, encoding) throws when fill is a number or a Buffer #8286 and confirm hex outputs: 050505, 000000, 070707, 616261

Note: Bazel was not available in this environment, so the buffer test target was not run locally.

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[umeraamir69]

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