Refactor AttributeGraphVendor to RawRepresentable struct

[Kyle-Ye]

Summary

• Refactor AttributeGraphVendor from a String-backed enum to a struct conforming to RawRepresentable and CaseIterable, aligning with the pattern used by RenderBoxVendor
• Add DocC comments for the type and each vendor case

Test plan

• Verify OPENATTRIBUTEGRAPH_COMPATIBILITY_TEST=0 swift build succeeds
• Verify OPENATTRIBUTEGRAPH_COMPATIBILITY_TEST=1 swift build succeeds
• Confirm downstream OpenSwiftUI builds without changes (API is source-compatible)

[augmentcode]

🤖 Augment PR Summary

Summary: Refactors AttributeGraphVendor from a String-backed enum into a RawRepresentable/CaseIterable struct to match the vendor-pattern used elsewhere.
Changes: Adds DocC documentation for the vendor type/cases and defines allCases explicitly for the known vendor identifiers.

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[augmentcode]

Review completed. 2 suggestions posted.

Comment augment review to trigger a new review at any time.

[augmentcode]

Changing AttributeGraphVendor from an enum to a struct drops the enum’s implicit Equatable/Hashable, which can break existing ==, Set/Dictionary usage, and switch constant patterns (e.g. case .oag). Consider explicitly conforming to Equatable/Hashable to preserve those behaviors.

Severity: medium

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[augmentcode]

AttributeGraphVendor(rawValue:) used to be failable when it was a RawRepresentable enum; making it non-failable can break downstream code that does optional binding (if let) or force-unwrapping on this initializer. If source-compatibility is a goal, consider also providing a failable init?(rawValue:) entry point.

Severity: medium

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 31.77%. Comparing base (91dd6f1) to head (eedc91a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/OpenAttributeGraphShims/OAGShims.swift	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #222      +/-   ##
==========================================
- Coverage   32.08%   31.77%   -0.32%     
==========================================
  Files          70       71       +1     
  Lines        2596     2628      +32     
==========================================
+ Hits          833      835       +2     
- Misses       1763     1793      +30     

Flag	Coverage Δ
ios	32.14% <66.66%> (+0.03%)	⬆️
macos	9.51% <66.66%> (-0.44%)	⬇️
ubuntu	12.03% <66.66%> (+0.24%)	⬆️

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Files with missing lines	Coverage Δ
Sources/OpenAttributeGraphShims/OAGShims.swift	66.66% <66.66%> (ø)

... and 7 files with indirect coverage changes

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