feat(gradle-plugin): upload native debug symbols via symbol-sets upload

[cat-ph]

💡 Motivation and Context

Native crash capture (#659) needs the app's .so debug symbols uploaded at build time, the same way the plugin already uploads ProGuard mappings. This registers an uploadPostHogNativeSymbols<Variant> task that points posthog-cli symbol-sets upload (>= 0.7.32) at the variant's merged native libs intermediates: the unstripped libraries as built, before AGP strips them for packaging. The CLI scans the directory, uploads every library carrying debug info and a GNU build id, and triages the rest with guidance.

Wiring choices:

• Registered for every variant, independent of minification: native symbolication is orthogonal to JVM obfuscation, so the task also exists (and auto-runs) for non-minified builds.
• Auto-hooked to assemble/install/bundle only when the app module builds native code itself (externalNativeBuild configured). Apps that only bundle prebuilt .so files from dependencies can run the task explicitly; auto-uploading for every app with any bundled .so (e.g. every React Native app) felt too surprising for a first release.
• Skipped via onlyIf when the variant has no .so files at all.
• Reuses the existing CLI discovery, credential, and dotenv plumbing (PostHogCliExecTask), so tasks.withType<PostHogCliExecTask> configuration applies to it like the mapping upload.

💚 How did you test it?

• ./gradlew -p posthog-android-gradle-plugin build green on a remote linux box (the plugin has no test suite today; the task follows the existing PostHogUploadProguardMappingsTask structure).
• Not yet done (why this is a draft): exercising the task against a real NDK app build and verifying the upload lands, planned as part of the end-to-end validation for the SDK PR.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes. (no test infrastructure exists in the plugin build; follows the existing upload task pattern)
• I updated the docs if needed. (docs PR is staged separately, gated on release)
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file