Add linear and radial gradients for shape fills

[scorsin-oai]

Description

This change adds support for fillGradient on the shape element.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Documentation improvement
• Performance optimization
• Test improvement
• Other (please describe)

Testing

• Tests pass locally (bazel test //...)
• Added/updated tests for changes (if applicable)
• Tested on multiple platforms (iOS/Android/Web/macOS as applicable)
• Manual testing performed (describe below)

Testing Details

Checklist

• Code follows project style guidelines
• Documentation updated (if needed)
• No breaking changes (or documented in description)
• Commit messages follow conventional format
• No secrets, API keys, or internal URLs included

Related Issues

Additional Context

[scorsin-oai]

Will add iOS impl before merging the PR

[github-actions]

⚠️ Bazel & CI Test Results

Test Suite	Result
Valdi Smoke Tests	✅ success
API Surface Check	✅ success
Snapshot Tests	✅ success
Linux: Build Compiler	✅ success
macOS: C++ & Platform Tests	✅ success
Linux: C++ Tests	✅ success
Linux: Build & Export	❌ failure

Some tests failed. Please check the workflow logs for details.

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