fix(#667): bound Parquet storage streaming

[cofin]

Summary

• make Arrow storage streaming explicitly Parquet-only and row-group-bounded
• stream local, fsspec, and obstore objects without draining obstore into BytesIO
• add deterministic reader cleanup, input validation, and consistent sync/async behavior

Details

All streaming contracts now expose keyword-only file_format="parquet" and batch_size=65_536. Validation occurs before storage access. Each backend iterates one Parquet row group at a time, and obstore uses open_reader() through a file-compatible proxy. The change also fixes base-path duplication.

Closes #667.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.83838% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.26%. Comparing base (a3a3012) to head (e129289).

Files with missing lines	Patch %	Lines
sqlspec/storage/backends/obstore.py	81.25%	7 Missing and 2 partials ⚠️
sqlspec/storage/backends/base.py	58.82%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #671   +/-   ##
=======================================
  Coverage   77.26%   77.26%           
=======================================
  Files         475      476    +1     
  Lines       67901    67972   +71     
  Branches     9335     9343    +8     
=======================================
+ Hits        52462    52517   +55     
- Misses      12035    12046   +11     
- Partials     3404     3409    +5     

Flag	Coverage Δ
integration	60.78% <40.40%> (-0.03%)	⬇️
py3.10	75.57% <82.82%> (-0.01%)	⬇️
py3.11	75.58% <82.82%> (+<0.01%)	⬆️
py3.12	75.58% <82.82%> (+<0.01%)	⬆️
py3.13	75.59% <82.82%> (+1.36%)	⬆️
py3.14	76.46% <83.83%> (+<0.01%)	⬆️
unit	64.79% <83.83%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
sqlspec/protocols.py	100.00% <100.00%> (ø)
sqlspec/storage/_arrow_stream.py	100.00% <100.00%> (ø)
sqlspec/storage/backends/fsspec.py	90.64% <100.00%> (+0.09%)	⬆️
sqlspec/storage/backends/local.py	81.50% <100.00%> (+0.18%)	⬆️
sqlspec/storage/backends/base.py	88.65% <58.82%> (-6.47%)	⬇️
sqlspec/storage/backends/obstore.py	83.12% <81.25%> (-0.89%)	⬇️

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