Fix engine-side KCL websocket authentication

[Joe-Spencer]

Summary

• forward the token already resolved by Zoo CLI as an in-band WebSocket Headers request before ExecKclProject
• tolerate only a bounded set of uncorrelated auth_token_missing responses that can race the authentication frame
• send ExecKclProject as named MessagePack binary while keeping authentication and ordinary modeling commands as JSON text

Why

ENGINE_EXECUTION=1 authenticated the WebSocket upgrade with the normal CLI credential, but OAuth tokens created by zoo auth login also need the protocol-level Authorization header frame. Without it, engine-side KCL execution could fail even though zoo auth status succeeded.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-features --all-targets -- -D warnings
• cargo test --locked context::test:: -- --nocapture (7 passed)
• cargo test --locked snapshot_via_engine -- --ignored --nocapture
• live unchanged five-file Axial Fan export with ENGINE_EXECUTION=1, all token environment variables removed, and authentication supplied only by the normal CLI credential store: succeeded in 6.03 s and produced an 889,607-byte STEP

[codecov]

Codecov Report

❌ Patch coverage is 96.61017% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.55%. Comparing base (540418d) to head (27bbff7).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/context.rs	96.61%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1816      +/-   ##
==========================================
+ Coverage   67.02%   67.55%   +0.52%     
==========================================
  Files          43       43              
  Lines        8403     8512     +109     
==========================================
+ Hits         5632     5750     +118     
+ Misses       2771     2762       -9     

Flag	Coverage Δ
unittests	67.55% <96.61%> (+0.52%)	⬆️

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