test: add config and priority fuzz tests

[YooLCD]

Summary

Adds native Go fuzz targets (Go 1.18+) to improve robustness
and support future OSS-Fuzz integration.

This PR contains the upstream fuzz targets.
The OSS-Fuzz integration PR will follow after this is merged.

Targets

• FuzzConfigYAML / FuzzConfigJSON / FuzzConfigTOML:
  Parser robustness against arbitrary and malformed input
• FuzzConfigPriority:
  Verifies Env > Config > Default priority correctness
  under arbitrary inputs

Results

All four fuzzers pass with zero crashes or panics.
Parallel-safe (uses t.Setenv), memory-bounded (1MB limit).

How to run

go test -fuzz=FuzzConfigYAML     -fuzztime=30s
go test -fuzz=FuzzConfigJSON     -fuzztime=30s
go test -fuzz=FuzzConfigTOML     -fuzztime=30s
go test -fuzz=FuzzConfigPriority -fuzztime=30s

Test output

ok  github.com/spf13/viper
ok  github.com/spf13/viper/internal/encoding/dotenv
ok  github.com/spf13/viper/internal/encoding/json
ok  github.com/spf13/viper/internal/encoding/toml
ok  github.com/spf13/viper/internal/encoding/yaml

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