fix: skip pruning for pre/code tags to preserve whitespace in code blocks

[Battleplus]

Fixes #2110. PruningContentFilter prunes whitespace-only spans inside pre/code tags, corrupting code blocks. Fix: skip pruning for pre and code tags where whitespace is significant.

[Battleplus]

Added a focused regression test that uses min_word_threshold to reproduce the prior removal of a whitespace-only syntax-highlighting span and asserts the span remains inside the code block. The new test passes. The existing full test_content_filter_prune.py file still has one unrelated baseline failure in test_threshold_types (fixed and dynamic currently both produce one block); the added regression itself is green. Commit: ee66b90.

[Battleplus]

Closing as a duplicate after the full audit. Earlier PR #2111 implements the same pre/code subtree guard and already includes substantially broader regression coverage for code whitespace preservation. The focused test added here passed, but consolidating review on #2111 is preferable.