[registry] Add SVG writing support for relationships during registration and generation

[Copilot]

Description

This PR fixes #837

Adds support for writing SVGs to disk when registering/generating relationships, mirroring existing component behavior.

Changes:

• Registration (models/registration/register.go): Write relationship SVGs (SvgColor, SvgWhite, SvgComplete) to disk and replace inline data with file paths, consistent with model/component handling
• Generation (registry/component.go):
  • CreateRelationshipsMetadataAndCreateSVGsForMDXStyle() - MDX docs
  • CreateRelationshipsMetadataAndCreateSVGsForMDStyle() - MD docs
  • getSVGForRelationship() - Extract SVGs from styles JSON with model fallback
• Tests: Coverage for SVG extraction, file creation, and model fallback scenarios

Example usage:

// Relationship SVGs now written during registration
if rel.Metadata != nil && rel.Metadata.Styles != nil {
    rel.Metadata.Styles.SvgColor, rel.Metadata.Styles.SvgWhite, svgCompletePath = 
        WriteAndReplaceSVGWithFileSystemPath(...)
}

Notes for Reviewers

Pre-existing build issue in github.com/meshery/schemas dependency (design_conversion.go:176 type mismatch) prevents full test execution. Unrelated to this PR.

Signed commits

• Yes, I signed my commits.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add support for writing svgs to disk when registering / generating relationships similar to components</issue_title>
<issue_description>### Current Behavior

Desired Behavior

Implementation

Acceptance Tests

Mockups

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

• Fixes Add support for writing svgs to disk when registering / generating relationships similar to components #837

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