[typespec-ts] Fix platform import issue for customization

[Copilot]

PR #4585 removed the #platform/ subpath import mechanism and static platform-types helpers in favor of direct external dependencies, but left four tests expecting the old behavior, causing the "Run unit tests" CI job to fail.

Source changes (from PR #4585)

• Remove PlatformTypeHelpers static helper (platform-types.ts + browser/react-native variants) and getBinaryStreamResponse static helper; promote both NodeReadableStream and getBinaryStreamResponse to AzureCoreDependencies as proper external dependencies from @azure/core-rest-pipeline / @azure-rest/core-client
• Remove #platform/ import generation from binder — drop useSubpathImports implementation in BinderImp (interface field kept for backward compat)
• Remove imports field (#platform/* subpath map) from getEntryPointInformation in package-common.ts; updatePackageFile continues to inject it for existing-package migrations
• Bump dependency versions: @azure-rest/core-client → ^2.7.0, @azure/core-rest-pipeline → ^1.24.0 (always overwritten on updatePackageFile)

Test fixes

• Remove imports property assertions from the two buildPackageFile ESM entrypoint tests — buildPackageFile no longer emits imports
• Update should migrate @azure/core-client expected @azure/core-rest-pipeline version: ^1.19.1 → ^1.24.0 (now always overwritten)
• Remove should use #platform subpath imports binder test — feature is gone