Add SPDX-License-Identifier to C# file headers

[Copilot]

License compliance tools (e.g., FOSSology) cannot resolve URL-based license references in file headers, making automated license scanning unreliable. Replacing with standard SPDX identifiers enables tooling to identify the license without network access or URL resolution.

Changes

• Replaced // <license>https://...LICENSE.md</license> with // SPDX-License-Identifier: MIT across 148 C# source files
• Covers files originally from NJsonSchema, NSwag, and MyToolkit — all MIT licensed by Rico Suter
• Left .NET Foundation-originated files unchanged (already carry proper MIT attribution)

Before:

// <license>https://github.com/RicoSuter/NJsonSchema/blob/master/LICENSE.md</license>

After:

// SPDX-License-Identifier: MIT

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Better license statement</issue_title>
<issue_description>You are creating great software, thank you very much for that.

An improvement could be a better display of the license for each file.
I am a license compliance expert working for Siemens. We scan all open source we use
to know all licenses, for example with tools like FOSSology (https://www.fossology.org/).

These tools have a hard time analyzing your file header, because they are not prepared to resolve
a URL like https://github.com/RicoSuter/NJsonSchema/blob/master/LICENSE.md.

The standard for license compliance would be an SPDX identifier like SPDX-License-Identifier: MIT,
see https://spdx.dev/ids/ for details. If you want to do a perfect job, you could also have a look at
https://reuse.software/.

I have my own OSS projects, so I know that updating each file costs some effort ... but it would make
your projects a little be mote perfect and better to use.</issue_description>

Comments on the Issue (you are @copilot in this section)

@RicoSuter Can you propose a header to be added in each C# file? Why are you even scanning source code files and not just use the license of the pkg?

• Fixes Better license statement #1610

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