feat: export DefaultDecodeHook for composition

[Solaris-star]

Summary

WithDecodeHook / DecodeHook replace Viper's built-in mapstructure hooks entirely. The default weak string-to-slice hook is unexported, so apps that add a custom hook silently lose duration/slice conversions and cannot restore them (#2139).

Fix

Export DefaultDecodeHook() and use it as the internal default so callers can compose:

WithDecodeHook(mapstructure.ComposeDecodeHookFunc(
    viper.DefaultDecodeHook(),
    myCustomHook,
))

Tests

go test . -count=1 -run TestDefaultDecodeHookComposable
# ok

Fixes #2139

[CLAassistant]

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