Add error handling to SDK version function

[dru1x]

Summary

Adds error handling to cover edge cases that might result in the SDK version caching being skipped and fatal errors being emitted.

Changes

• Replaced the function-local static $version cache with class-level self::$sdkVersion/self::$sdkVersionResolved properties, so "has this been computed" is tracked independently of the value itself.
• A falsy/null result is now cached correctly instead of being recomputed on every call.
• Extracted the resolution logic into resolveSdkVersion(string $composerJsonPath): string, guarded with is_readable() and a try/catch (Throwable) around the JSON_THROW_ON_ERROR decode and InstalledVersions::getPrettyVersion() call, falling back to 'unknown' instead of emitting a fatal error.

Testing

Added to tests/Unit/ExpoPushConnectorTest.php:

• resolveSdkVersion() still resolves the real installed version from the project's actual composer.json.
• Falls back to 'unknown' when composer.json is missing, or malformed.
• sdkVersion() returns a cached value without recomputing, including a cached falsy value ('') — the direct regression test for the caching bug.

Related Issues

Fixes #90