Bump version to 6.5.1 and enable apple-generic versioning

[Kyle-Ye]

Summary

• Update CURRENT_PROJECT_VERSION to 6.5.1 and enable VERSIONING_SYSTEM = apple-generic in xcconfig
• Guard manual OpenAttributeGraphVersionNumber/VersionString symbols with #if SWIFT_PACKAGE to avoid duplicate symbols when building with Xcode's auto-generated version object
• Update version number from 6.0 to 6.5 in OAGVersion.c and compatibility tests

Test plan

• Verify macOS/iOS CI builds pass
• Verify compatibility tests pass with updated version expectations
• Verify Xcode project builds without duplicate symbol errors

[augmentcode]

🤖 Augment PR Summary

Summary: Bumps OpenAttributeGraph build/version metadata to 6.5.1 and enables Xcode’s apple-generic versioning.
Changes: Guards manual OpenAttributeGraphVersionNumber/String behind #if SWIFT_PACKAGE and updates compatibility tests to expect version 6.5.

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[augmentcode]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.08%. Comparing base (e970392) to head (e648790).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #216 +/- ##
==========================================
+ Coverage 32.01% 32.08% +0.07% 
==========================================
Files 70 70 Lines 2596 2596 ==========================================
+ Hits 831 833 +2 + Misses 1765 1763 -2 

Flag	Coverage Δ
ios	32.10% <ø> (+0.07%)	⬆️
macos	9.95% <ø> (ø)
ubuntu	0.00% <ø> (ø)

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see 1 file with indirect coverage changes

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