ref(service): Standardize backend session token codecs

[lcian]

Backends can now define a sized session-token representation and centralize its codec while the
object-safe resumable API continues passing an opaque BackendToken. GCS owns the first typed
implementation and decodes tokens at each resumable operation boundary; decorators and backends
without resumable support continue forwarding or rejecting opaque tokens as before.

BackendToken is now a serde-transparent newtype, so its JSON representation, encrypted session
envelope, and GCS total_length.session_uri encoding remain wire-compatible. This PR is stacked on
#615, which provides the generalized encryption boundary used by these tokens.

A disadvantage of this approach is that the backend-specific types need to become pub, as they become part of the Backend API which is pub itself.

[codecov]

Codecov Report

❌ Patch coverage is 68.42105% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.15%. Comparing base (28141dd) to head (99c42b0).

Files with missing lines	Patch %	Lines
objectstore-service/src/backend/common.rs	0.00%	12 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                          @@
##           ref/generalize-encryption-cipher     #616      +/-   ##
====================================================================
- Coverage                             89.20%   89.15%   -0.06%     
====================================================================
  Files                                   110      110              
  Lines                                 19036    19057      +21     
====================================================================
+ Hits                                  16981    16990       +9     
- Misses                                 2055     2067      +12     

Components	Coverage Δ
Rust Backend	92.84% <68.42%> (-0.08%)	⬇️
Rust Client	81.97% <ø> (ø)
Python Client	93.56% <ø> (ø)

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[lcian]

bugbot run
@sentry review

[lcian]

I'm not sure if this change is beneficial, wdyt @jan-auer?

[cursor]

✅ Bugbot reviewed your changes and found no new issues!

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^{Reviewed by Cursor Bugbot for commit 99c42b0. Configure here.}