Fix TS_2DIFF float/double page compatibility

[hongzhi-gao]

Summary

• Fix C++ TS_2DIFF float/double page layout to match Java overflow-page encoding.
• Fix Java table-read time decoder selection to use actual time chunk encoding (instead of global default), preventing mis-decoding on PLAIN time chunks.
• Harden googletest zip handling in CMake: reject/remove empty or invalid zip files and only accept valid ZIP magic before enabling tests.

Encoding Layout

Legacy raw block (old C++ path):
[ TS_2DIFF inner block ]

Java-compatible overflow page (fixed C++ path):
[ outer_magic(varint=2147483646) ]
[ count(varint) ]
[ bitmap_under ]
[ bitmap_over ]
[ inner_max_point(varint=2) ]
[ TS_2DIFF inner block payload ]

Test Input

Float values: 3.123456768E20f, NaN
Expected encoded hex (Java baseline):

FE FF FF FF 07 02 00 03 02 00 00 00 01 00 00 00 00 1E 38 8A AA 61 87 75 56

Verification

• C++ encoded output matched the Java baseline byte-for-byte.
• Java table query can read both C++-generated and Java-generated files

[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.00508% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.67%. Comparing base (5e2130f) to head (13bb80e).

Files with missing lines	Patch %	Lines
cpp/src/encoding/ts2diff_decoder.h	55.55%	36 Missing ⚠️
cpp/src/encoding/ts2diff_encoder.h	74.10%	29 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #796      +/-   ##
===========================================
- Coverage    62.74%   62.67%   -0.08%     
===========================================
  Files          706      707       +1     
  Lines        42842    42858      +16     
  Branches      6326     6376      +50     
===========================================
- Hits         26882    26860      -22     
- Misses       14968    15006      +38     
  Partials       992      992              

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