fix(#695): add backend-neutral URI resolution

[cofin]

Summary

Object-store backends exposed backend-native paths but no portable way to format the final local path or remote URI. This made consumers manually join backend attributes, which duplicated URI prefixes with obstore.

• add resolve_uri() to the public object-store contract and built-in backends
• preserve existing backend path and registry behavior while returning equivalent fsspec and obstore addresses
• document unsigned URI resolution and the custom-backend compatibility requirement

Closes #695.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.40%. Comparing base (ea00178) to head (c6baab6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #697   +/-   ##
=======================================
  Coverage   77.40%   77.40%           
=======================================
  Files         486      486           
  Lines       68287    68307   +20     
  Branches     9406     9410    +4     
=======================================
+ Hits        52856    52876   +20     
- Misses      12020    12023    +3     
+ Partials     3411     3408    -3     

Flag	Coverage Δ
integration	60.85% <55.00%> (+<0.01%)	⬆️
py3.10	75.72% <100.00%> (+0.01%)	⬆️
py3.11	75.73% <100.00%> (+<0.01%)	⬆️
py3.12	75.73% <100.00%> (+11.00%)	⬆️
py3.13	75.73% <100.00%> (+0.01%)	⬆️
py3.14	76.61% <100.00%> (+<0.01%)	⬆️
unit	65.05% <100.00%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
sqlspec/protocols.py	100.00% <ø> (ø)
sqlspec/storage/backends/base.py	88.65% <ø> (ø)
sqlspec/storage/backends/fsspec.py	92.30% <100.00%> (+1.66%)	⬆️
sqlspec/storage/backends/local.py	81.68% <100.00%> (+0.18%)	⬆️
sqlspec/storage/backends/obstore.py	83.65% <100.00%> (+0.52%)	⬆️

... and 2 files with indirect coverage changes

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