Remove the in-process worker, port Netherite to isolated, rebuild CI

[Viswanath (ViswanathBalusu)]

The .NET in-process backend was a parallel fork of the isolated one, not a consumer of it, so removing it touches nothing the isolated projects rely on.

Removed:
durablefunctionsmonitor.dotnetbackend
tests/durablefunctionsmonitor.dotnetbackend.tests
custom-backends/{mssql,netherite,netcore21,netcore31}

Netherite was the only storage provider without an isolated backend, so it is ported rather than dropped: durablefunctionsmonitor.dotnetisolated.netherite plus custom-backends/dotnetIsolated-netherite, mirroring the .mssql pair. Task Hub discovery reads Netherite's DurableTaskPartitions table via core's TableClient, which also gains identity-based Storage support that the in-process version lacked. Reaching that helper needs one InternalsVisibleTo, matching how core already exposes internals to its test project.

VS Code extension now launches isolated backends: FUNCTIONS_WORKER_RUNTIME is dotnet-isolated and the contradictory --csharp flag is gone. The Functions v4 version gate moved ahead of the custom-backend dispatch, since every backend now requires it. The backendVersionToUse setting is dropped; it only ever selected .NET Core 2.1/3.1 in-process backends.

Custom backends reference durablefunctionsmonitor.dotnetisolated.* by project, so they cannot build from inside the VSIX. CI now pre-publishes them as binaries instead of shipping source, which also removes the slow dotnet publish on first launch. Docker builds move to a repo-root context for the same reason.

CI/CD:
build.yml reusable build, called by all publishing workflows
pr-build.yml PR validation, publishes nothing
main-build.yml main -> GitHub Packages prerelease
publish-nuget.yml tag v* -> nuget.org via Trusted Publishing (OIDC)
push-to-nuget.yml deleted, superseded

Directory.Build.props becomes the single source of version truth, replacing 12 hardcoded copies. Package ids and versions are now nuspec tokens supplied at pack time.

Fixes a latent packaging bug: MSBuild only auto-imports build/.targets, but the nuspecs shipped a fixed filename, so the targets that copies DfmStatics into a consumer's output never fired for DurableFunctionsMonitor.DotNetIsolated.MsSql. Now targeted as build/$id$.targets.

publish-nuget.yml hard-fails unless PACKAGE_ID_PREFIX is set, because DurableFunctionsMonitor.DotNetIsolated and .MsSql are owned by other accounts on nuget.org and cannot be published from this repo.