[WIP] Migrate changes from autorest.typescript to typespec-azure

[Copilot]

• Inspect PR fix: replace NodeJS.ReadableStream with platform-conditional NodeReadableStream in RLC generated code #3998 and identify the exact files/behavior to migrate into packages/typespec-ts
• Run baseline build/lint/test commands to capture current repository state before edits
• Apply minimal source changes in packages/typespec-ts matching PR fix: replace NodeJS.ReadableStream with platform-conditional NodeReadableStream in RLC generated code #3998 behavior
• Update focused unit tests for the migrated behavior
• Run targeted tests for changed areas and confirm they pass
• Run security scan (CodeQL checker) and address any findings

Original prompt

could you create a new pr to migrate this pr(#3998 )'s changes
to
https://github.com/Azure/typespec-azure/tree/main/packages/typespec-ts

The user has attached the following file paths as relevant context:

• .github\copilot-instructions.md

Created from VS Code.