Add MakeClientFromApi to C++ PJRT bridge

[Dale-Black]

Summary

Adds a new C++ function MakeClientFromApi that allows Julia-allocated PJRT_Api structs to be registered with XLA's PJRT infrastructure without needing dlopen on a shared library.

This is the C++/Bazel portion of the Metal PJRT backend work, split out per @avik-pal's request from #2489 so a new JLL can be built independently.

Changes

• deps/ReactantExtra/API.cpp (+21 lines): New MakeClientFromApi(api, device_type, client_name, error) function that:
  1. Calls pjrt::SetPjrtApi() to register the API struct
  2. Calls InitializePjrtPlugin() to initialize the plugin
  3. Calls RegisterProfiler() and returns the client via GetCApiClient()
• deps/ReactantExtra/BUILD (+1 line): Exports _MakeClientFromApi symbol

Context

The Metal PJRT backend (PR #2489) implements PJRT callbacks as Julia @cfunction pointers packed into a PJRT_Api struct. Unlike GPU/TPU plugins which are loaded via dlopen on a .so/.dylib, the Metal backend lives in Julia and needs a way to register its PJRT_Api struct directly with XLA. This function provides that bridge.

Test plan

• JLL builds successfully with the new symbol exported
• nm libReactantExtra.so | grep MakeClientFromApi shows the symbol
• Full Metal backend integration tested in Metal PJRT backend via MPSGraph + pure-Julia plugin #2489

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.10%. Comparing base (b39a1fc) to head (5489117).
⚠️ Report is 700 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (b39a1fc) and HEAD (5489117). Click for more details.

HEAD has 92 uploads less than BASE

Flag	BASE (b39a1fc)	HEAD (5489117)
	99	7

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2490       +/-   ##
===========================================
- Coverage   68.16%   38.10%   -30.06%     
===========================================
  Files         109      196       +87     
  Lines       11779    29872    +18093     
===========================================
+ Hits         8029    11384     +3355     
- Misses       3750    18488    +14738     

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