Fix NameStartsWith filter not matching numeric item names

The NameStartsWith, NameStartsWithOrGreater, and NameLessThan API
filters compare user input against the internal SortName, which
zero-pads numbers to 10 digits for natural sorting. This caused
numeric filters like NameStartsWith=1 to fail because "1" doesn't
match the padded sort name "0000000012...".

Normalize filter input through ModifySortChunks before comparison so
that padding is applied consistently. For NameStartsWith, add a
TrimStart('0') fallback so that numeric prefixes like "1" correctly
match sort names such as "0000000012 years a slave".

Add NormalizeSortNameFilter helper on BaseItem and make
ModifySortChunks public. Add unit tests covering numeric, alphabetic,
and mixed prefix matching for all three filter types.

Fixes #1470

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