fix(service): Honor byte range requests in Bigtable backend

[jan-auer]

The Bigtable backend ignored the ByteRange parameter in get_tiered_object (the parameter was named _range), returning the full object with no ContentRange regardless of what the client requested. This meant clients always received 200 with the complete payload instead of 206 Partial Content.

Adds an apply_range helper that resolves the byte range against the payload and produces a zero-copy Bytes::slice, then wires it into the get_tiered_object return path.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.47%. Comparing base (c905833) to head (b7bbeec).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #506      +/-   ##
==========================================
+ Coverage   87.38%   87.47%   +0.08%     
==========================================
  Files          86       86              
  Lines       13825    13912      +87     
==========================================
+ Hits        12081    12169      +88     
+ Misses       1744     1743       -1     

Components	Coverage Δ
Rust Backend	92.26% <100.00%> (+0.06%)	⬆️
Rust Client	80.06% <ø> (ø)
Python Client	88.51% <ø> (ø)

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[lcian]

LGTM