[gpu] Add acquire/release context accessors to all GPU backends

[derek-gerstmann]

Summary

The per-backend halide_*_acquire_context() / halide_*_release_context() functions can't be overridden on Windows: the runtime-library symbols are strong and are only referenced internally to the runtime libs, so the weak-symbol replacement that works on Linux/macOS doesn't apply there. The CUDA backend already provides runtime accessors (halide_set_cuda_acquire_context() / halide_set_cuda_release_context()) that swap a function pointer instead, which works on all platforms.

This PR brings the OpenCL, Metal, D3D12Compute, Vulkan, and WebGPU backends in line with CUDA.

Changes

• Runtime: each backend's acquire/release context function is split into a halide_default_* implementation plus a dispatcher that calls through a settable pointer, with new halide_set_*_acquire_context / halide_set_*_release_context accessors and matching typedefs in the public runtime headers.
• JIT: new JITHandlers fields, merge_handlers entries, per-context handler shims, and make_module() hookup, mirroring the CUDA path.
• runtime_api.cpp: register the new accessors — and the CUDA ones, which had been missing — in halide_runtime_api_functions[].
• Tests: test/correctness/custom_{opencl,metal,d3d12compute,vulkan,webgpu}_context.cpp, modeled on custom_cuda_context.cpp.

Testing

Full build passes. The Metal test passes end-to-end on an Apple GPU (HL_JIT_TARGET=host-metal); the other backends' tests [SKIP] unless their target is enabled.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 18.65672% with 109 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.11%. Comparing base (a3690b3) to head (7fdd70d).

Files with missing lines	Patch %	Lines
src/JITModule.cpp	18.65%	93 Missing and 16 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9431      +/-   ##
==========================================
- Coverage   70.12%   70.11%   -0.02%     
==========================================
  Files         261      261              
  Lines       79405    79539     +134     
  Branches    19362    19397      +35     
==========================================
+ Hits        55684    55765      +81     
- Misses      17896    17958      +62     
+ Partials     5825     5816       -9     

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