Add GetContentType() to Attachment API for retrieving OPC content types

[Copilot]

lib3mf was parsing [Content_Types].xml internally but discarding the data, making it impossible for API consumers to determine the MIME type of package attachments (e.g., distinguishing PNG from JPEG thumbnails).

Changes

OPC layer

• COpcPackageReader::readContentTypes() now populates two maps: m_ContentTypes (Override entries, keyed by exact part path) and m_ContentTypesByExtension (Default entries, keyed by extension)
• New getContentType(path) method: checks Override entries first, falls back to extension-based lookup
• IOpcPackageReader interface and CKeyStoreOpcPackageReader updated accordingly

Model layer

• CModelAttachment gains a m_sContentType field with getContentType()/setContentType() accessors
• NMR_ModelReader_3MF_Native populates content type when loading thumbnail, texture, and custom attachments

API / bindings

• IAttachment::GetContentType() added as a pure virtual, returning the content type string (empty for programmatically-created attachments not read from a package)
• C ABI: lib3mf_attachment_getcontenttype
• Updated: C++ implicit binding, C++ dynamic binding (typedef, wrapper table, load/lookup)

Usage

auto reader = model->QueryReader("3mf");
reader->ReadFromFile("model.3mf");

auto thumbnail = model->GetPackageThumbnailAttachment();
std::string ct = thumbnail->GetContentType(); // e.g. "image/png" or "image/jpeg"

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add a way to retrieve Attachment Content Types</issue_title>
<issue_description>Currenty lib3mf does not parse the [Content_Types].xml in the package.
It would be great to properly parse this and retrieve content types of Package Parts via Lib3MF API.

Use cases are plenty, but for example:
At the moment it is hard to distinguish package thumbnails, if they are meant to be PNG or JPEGs.</issue_description>

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

• Fixes Add a way to retrieve Attachment Content Types #379

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