Fix custom prefixes in BertWordPieceTokenizer

[HaokaiDing]

Fixes #2412.

BertWordPieceTokenizer passes wordpieces_prefix to its decoder but leaves the WordPiece model using ##. Loading a vocabulary trained with a custom prefix such as @@ consequently turns known words into [UNK].

Forward the prefix to both WordPiece constructor branches. Add tests for default/custom prefixes that check tokens, IDs, offsets and decoding, plus a training/save/reload round trip.

Validation:

• Before the fix: 2 failed, 3 passed. After: 5 passed, 2 network tests deselected in tests/implementations/test_bert_wordpiece.py.
• Tested the current checkout's Python package with the released tokenizers 0.23.2 native extension; the local runner verifies the wrapper's source path and native module identity. Current Rust was not rebuilt and network-dependent tests were not run.
• Ruff lint and format checks pass on both changed files.
• Python 3.13.15 on macOS ARM64. No dependency changes.

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