Improve declared license collection from nuspec files (#2669)

[teyesan]

Description

Improve NuGet .nuspec license handling to correctly process the different ways license information can be declared.

Changes

• Handle <license type="expression"> as the package's declared license expression.
• Handle <license type="file"> as a reference to a license file and store the filename in extra_data["license_file"].
• Preserve support for the legacy <licenseUrl> field when no <license> element is present.
• Ignore NuGet's deprecated placeholder licenseUrl (https://aka.ms/deprecateLicenseUrl).
• Update NuGet package test fixtures and expected results to cover license expressions and license files.

License precedence

When <license type="expression"> is present, the SPDX expression is used as the declared license statement.

When <license type="file"> is present, the referenced filename is stored separately in extra_data["license_file"], since it's a reference rather than license text itself, not the declared license statement directly.

A real (non-deprecated) <licenseUrl> is still treated as meaningful license information: it fills in the declared license statement when nothing else is available, or is appended alongside an SPDX expression when both are present. This preserves useful license info from licenseUrl rather than discarding it whenever a <license> element also exists.

NuGet's deprecated placeholder value (https://aka.ms/deprecateLicenseUrl) is always ignored, regardless of what else is present.

Fixes #2669

Tasks

• Reviewed contribution guidelines
• PR is descriptively titled 📑 and links the original issue above 🔗
• Tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR
  Run tests locally to check for errors.
• Commits are in uniquely-named feature branch and has no merge conflicts 📁
• Updated documentation pages (if applicable)
• Updated CHANGELOG.rst (if applicable)

[AyanSinhaMahapatra]

@teyesan there are no test files for your tests.

[teyesan]

Two tests are designed to fail currently and will pass once fix has been pushed.