[v1][Spec Decode] Make sliding window compatible with eagle prefix caching

[heheda12345]

#17137 drops the last matched block to support eagle. This strategy is not correct for sliding window layers.

When sliding window size is 4 and block_size is 2, we need at least 2 blocks. However,
find_longest_cache_hit of sliding_window_layers will return [NULL_BLOCK, NULL_BLOCK, BLOCK_A, BLOCK_B], and the blocks are changed to [NULL_BLOCK, NULL_BLOCK, BLOCK_A] due to eagle, which is not correct.

To fix the bug, this pr moves the logic to handle eagle into specialized_manager. This pr also fixes an outdated comment in test_eagle_enabled_removes_last_block

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[WoosukKwon]

LGTM! Thanks for finding the bug and fixing it.