Fix equirectangular typo in generate_spherical_xml default

[Hashim1999164]

Summary

• Corrects the default projection value in generate_spherical_xml from equiretangular to equirectangular.
• Without this, callers that omit projection (GUI and docker paths) fail the equality check and inject non-spherical metadata.
• Adds unit tests covering the default and none projection paths.

Fixes #336

Test plan

• python3 -m unittest spatialmedia_test.TestGenerateSphericalXml -v
• Confirm GUI/docker inject paths now emit equirectangular spherical XML by default

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