[Feature/Testing]: Add native Go fuzzing target for urunc system configuration loading and validation

[ParthMozarkar]

Summary

urunc parses its system configuration file (/etc/urunc/urunc.json or custom path) via LoadUruncConfig() and UruncConfig.Validate() in pkg/unikontainers/urunc_config.go.

Currently:

1. urunc_config_test.go contains static unit tests for valid JSON formats, but does not systematically test malformed JSON structures, invalid base64 string paths, or out-of-range memory/vCPU integers.
2. The repository lacks automated Go native fuzzing (go test -fuzz) to detect unexpected panics or unhandled errors when processing arbitrary configuration payloads (aligning with Add fuzzing and robustness testing #852).

Expected Behavior

• pkg/unikontainers should include a native Go fuzzing target FuzzLoadUruncConfig(f *testing.F) that generates random byte payloads to fuzz JSON decoding and validation.
• Validated configurations should guarantee clean error returns without triggering nil-pointer panics or slice bounds errors on Validate().

Proposed Changes

• Expose a memory-slice helper ParseUruncConfigBytes(data []byte) (*UruncConfig, error) in pkg/unikontainers/urunc_config.go.
• Implement FuzzLoadUruncConfig(f *testing.F) in pkg/unikontainers/urunc_config_test.go with initial seed corpuses for valid JSON structures.
• Verify invariant execution using go test -v -fuzz=FuzzLoadUruncConfig -fuzztime=30s ./pkg/unikontainers.