NativeAOT: Use lib prefix by default on Unix for native library outputs

[Copilot]

Description

On Unix, the lib prefix (e.g., libfoo.so, libfoo.dylib, libfoo.a) is the standard convention for shared and static libraries. Previously, NativeAOT only applied this prefix when the internal _UseNativeLibPrefix property was explicitly set — Android tests required it, but other Unix targets didn't get it by default.

This change makes the lib prefix the default for non-executable NativeAOT outputs on all non-Windows targets, and introduces a public UseNativeLibPrefix property for opting out.

Changes

• Microsoft.NETCore.Native.targets: Add public UseNativeLibPrefix property defaulting to true on non-Windows. Replace the private _UseNativeLibPrefix check in the NativeBinaryPrefix condition with the new property (the _targetOS != 'win' guard is preserved so Windows builds are unaffected even with an explicit opt-in).
• eng/testing/tests.android.targets: Remove the now-redundant <_UseNativeLibPrefix>true</_UseNativeLibPrefix> — the default behavior covers this.

Opt-out

Projects that need to suppress the prefix can set:

<UseNativeLibPrefix>false</UseNativeLibPrefix>

Changes to public API surface

None — MSBuild property changes only. UseNativeLibPrefix is the new public opt-out property.

Testing

Existing Android NativeAOT test infra validates the lib-prefixed output; no behavior change there. The main impact is that Unix NativeAOT library outputs now follow the platform convention by default.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Use correct native lib prefix in native AOT for android</issue_title>
<issue_description>#118299 introduced _UseNativeLibPrefix as an underscored property. Looks like android may need to set this: dotnet/android#10822.

At the time we didn't want to change the default to avoid breaking existing scenarios, but I wonder - could we try using the lib prefix by default on unix and take this breaking change? AFAIK this is a very common convention on unix so it would probably be a good default.

We could also introduce a public property to allow opting out for scenarios that don't want this.

@jtschuster @MichalStrehovsky </issue_description>

<agent_instructions>Use the native 'lib' prefix by default on unix.</agent_instructions>

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

• Fixes Use correct native lib prefix in native AOT for android #124610

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