Construct server engines on their owner worker

[justinchuby]

Summary

• construct each Engine inside its single owner worker and return a typed readiness result
• remove EngineOwner and forbid unsafe code in the server crate
• propagate initialization failures/panics/startup failures while preserving joined teardown, registry unload/drop, and in-flight handles
• keep model parsing and other Send-safe load data outside the worker

Scope

This is the owner-thread lifecycle prerequisite only. It preserves the #2056 global lease/session routing and #2044 state split. W = 1; this PR makes no concurrency, parallel execution, or batching claim.

Tests

• cargo test -p onnx-genai-server --lib (319 passed, 2 ignored)
• cargo test -p onnx-genai-server --test http (40 passed, 1 ignored)
• cargo test -p onnx-genai-server --lib session (50 passed)
• cargo test -p onnx-genai-server --lib unload (7 passed)
• targeted worker initialization and engine lifecycle tests
• cargo fmt --all -- --check
• cargo clippy -p onnx-genai-server --all-targets -- -D warnings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.14%. Comparing base (6caef22) to head (2513f1f).
⚠️ Report is 40 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2111      +/-   ##
==========================================
+ Coverage   80.30%   81.14%   +0.84%     
==========================================
  Files         426      429       +3     
  Lines      205244   215150    +9906     
  Branches   205244   215150    +9906     
==========================================
+ Hits       164811   174578    +9767     
- Misses      34781    34797      +16     
- Partials     5652     5775     +123     

Flag	Coverage Δ
cli-ort-linux	72.51% <ø> (?)
cli-ort-windows	72.01% <ø> (-0.10%)	⬇️
mlas	86.02% <ø> (?)
offline	81.28% <ø> (+0.76%)	⬆️

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see 65 files with indirect coverage changes

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