chore: Migrate dotnet-sdk-internal into dotnet-core

[jsonbailey]

Overview

Migrates the LaunchDarkly.InternalSdk package from the standalone dotnet-sdk-internal repo into this monorepo at pkgs/shared/internal/, following the pattern used for dotnet-sdk-common (#105) and its follow-up corrections (#117, #121, #123, #124).

Resolves SDK-2572.

Intentionally a chore: so release-please does not cut a release for the migration itself. The manifest is seeded at the current version (3.8.0); the first real release happens on the next feat/fix touching the package.

What's included

• pkgs/shared/internal/ — src/ and test/ copied verbatim from the source repo (net8 work from dotnet-sdk-internal chore(main): release LaunchDarkly.ServerSdk.Ai 0.6.0 #69, released as 3.8.0), plus README / CONTRIBUTING / CHANGELOG / PROVENANCE / .sln and the package's own public key LaunchDarkly.InternalSdk.pk.
• Signing preserved — github_actions.env sources the same S3 key the package uses today (launchdarkly-releaser/dotnet/LaunchDarkly.snk), so the assembly's strong-name public-key token is unchanged. The only csproj change is repointing AssemblyOriginatorKeyFile to the new repo-relative path.
• Release/CI wiring — github_actions.env, docfx.json, CI workflow (server-shared-internal.yml), release-please config + manifest entries, and release.yml / release-please.yml jobs. Built on common's post-fix baseline, so it already incorporates the naming (chore: Fix naming of common packages for release please #117), release-type: simple (chore: Set release-please project type #121), unquoted key-pair (chore: Update assembly key path pair #123), and docfx (chore: Add missing docfx configuration files #124) corrections.

Not included (deliberately)

• No consumer changes. The server/client SDKs keep their published LaunchDarkly.InternalSdk PackageReference; bumping them to a monorepo-published version is a separate follow-up PR.
• The old repo is untouched (archival is tracked separately under SDK-2573).

Validation

• net8.0: 180/180 unit tests pass in the new location.
• netstandard2.0: builds clean. (net462 is Windows-only — covered by the CI matrix.)
• release-please-config.json / manifest are valid JSON; package-name matches the real <PackageId> and the extra-files csproj path resolves correctly.

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Note

Low Risk
Repo layout and CI/release wiring only; implementation is a verbatim import with unchanged signing and no consumer dependency changes in this PR.

Overview
This PR brings LaunchDarkly.InternalSdk into the monorepo at pkgs/shared/internal/, mirroring how shared common packages live here. Library source, tests, changelog, and package metadata are added under that path (seeded at 3.8.0 in .release-please-manifest.json).

Build and release plumbing is extended so the package is first-class in this repo: DotnetCore.sln nests the new projects, server-shared-internal.yml runs build/test on path changes (Ubuntu netstandard2.0 / Windows net462), and release-please.yml / release.yml gain a pkgs/shared/internal release path with provenance, aligned with other shared packages.

Signing is wired through github_actions.env to the same strong-name key as before; the csproj only changes the AssemblyOriginatorKeyFile path for the new layout. No server/client SDK PackageReference bumps are in this diff—those stay on the published NuGet until a follow-up.

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