fix: general block decompression mismatch for Lance 2.2 dictionaries

[Xuanwo]

PR #4900 enabled writing dictionary pages through Compression::General, but the read path DefaultDecompressionStrategy::create_block_decompressor still routed those pages directly into BinaryBlockDecompressor. When the compressed payload actually held fixed-width dictionary values (e.g., MiniBlockSchedulerDictionary with fixed Int8 lists), the first byte was reinterpreted as bits_per_offset, panicking inside BinaryBlockDecompressor.

We didn't detect this during PR because we didn't add the test for 2.2 version.

This PR fixed this issue and added a new test to cover.

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This PR was primarily authored with Codex using GPT-5-Codex and then hand-reviewed by me. I AM responsible for every change made in this PR. I aimed to keep it aligned with our goals, though I may have missed minor issues. Please flag anything that feels off, I'll fix it quickly.

[westonpace]

Do we still need impl BlockDecompressor for CompressedBufferEncoder?

[Xuanwo]

Do we still need impl BlockDecompressor for CompressedBufferEncoder?

I think they don't need anymore, let me create a new PR to remove them.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.85714% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.74%. Comparing base (5f60351) to head (7a78a0a).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
rust/lance-encoding/src/compression.rs	70.17%	11 Missing and 6 partials ⚠️
...ust/lance-encoding/src/encodings/physical/block.rs	84.61%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5025   +/-   ##
=======================================
  Coverage   81.73%   81.74%           
=======================================
  Files         340      340           
  Lines      137432   137499   +67     
  Branches   137432   137499   +67     
=======================================
+ Hits       112332   112395   +63     
+ Misses      21377    21368    -9     
- Partials     3723     3736   +13     

Flag	Coverage Δ
unittests	81.74% <72.85%> (+<0.01%)	⬆️

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