Add automatic content hash for uploads

[AndreyVMarkelov]

Computes and sends a Dropbox content_hash for seekable upload streams so the server verifies upload integrity and rejects corrupted uploads. Applies to all upload routes whose request carries a ContentHash (direct upload and upload sessions).

Behavior

• On by default via DropboxClientConfig.AutoContentHash.
• Opt out per-client (AutoContentHash = false) or per-upload (ContentHasher.WithoutAutoContentHash(stream)).
• Non-seekable streams and manually supplied content hashes are left untouched.
• Cost: seekable uploads are read twice (once to hash, once to send). Documented in the README.

Implementation

• ContentHasher: public helper for the Dropbox block-SHA256 content hash (sync + async) and the opt-out wrapper.
• Handler hashes the body, restores the stream position, and sets the hash on a cloned request. Retry now rewinds to the original upload position instead of 0.
• Property access is resolved once per request type and cached.

Tests

• Unit tests for the hasher (known vectors, block boundary, async) and upload wiring (auto, manual-wins, opt-out, non-seekable, position preservation, config disable).
• Live e2e tests: direct upload round-trip and a chunked upload session, asserting the server-returned content hash matches and content round-trips.

[codecov]

Codecov Report

❌ Patch coverage is 71.11111% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.60%. Comparing base (0e0c773) to head (e1c7503).

Files with missing lines	Patch %	Lines
dropbox-sdk-dotnet/Dropbox.Api/ContentHasher.cs	73.80%	30 Missing and 3 partials ⚠️
...ox-sdk-dotnet/Dropbox.Api/DropboxRequestHandler.cs	67.34%	26 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #385      +/-   ##
==========================================
+ Coverage   59.38%   61.60%   +2.22%     
==========================================
  Files          16       17       +1     
  Lines        1433     1654     +221     
  Branches      136      155      +19     
==========================================
+ Hits          851     1019     +168     
- Misses        520      565      +45     
- Partials       62       70       +8     

Flag	Coverage Δ
integration	57.19% <62.22%> (+1.22%)	⬆️
unit	33.91% <70.22%> (+23.58%)	⬆️

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