Align dotnet hosted agent samples with the Python side (PR #5281) by
reorganizing the directory structure:
- Remove HostedAgentsV1 entirely (old API pattern)
- Split HostedAgentsV2 into invocations/ and responses/ based on protocol
- Move Using-Samples accordingly (SimpleAgent to responses, SimpleInvocationsAgent to invocations)
- Update slnx with new project paths and add previously missing invocations projects
- Update README cd paths from HostedAgentsV2 to invocations or responses
- Rename .env.local to .env.example to match Python naming convention
- Fix format violations in newly included invocations projects
