Fix CUDA arena cfg lifetime, add ROCm dispatch, harden VitisAI LoadLibrary, add missing cctype include

[Copilot]

Addresses review feedback from the EP decoupling PR.

• CUDA arena cfg use-after-free (src/cuda/session_options.cpp): arena_cfg was scoped inside the if (use_arena_management) block and destroyed before AppendExecutionProvider_CUDA_V2 consumed it. Moved declaration outside the block.

• ROCm path regression (new src/rocm/session_options.{h,cpp}): ROCm had no dispatch entry and would silently fall through to the generic V2/V1 path, which doesn't support AppendExecutionProvider_ROCM. Added a dedicated provider implementation matching the original behavior, wired into the dispatch table and cmake glob.

• VitisAI LoadLibrary (src/vitisai/session_options.cpp): LoadLibrary result was unchecked—GetProcAddress would be called with a null handle on failure. Added null-check with error, replaced C-style cast with reinterpret_cast.

• Missing <cctype> (src/openvino/session_options.cpp): RemoveAllWhitespace calls std::isspace without including <cctype>, relying on transitive includes.

// Before: arena_cfg destroyed before use
if (use_arena_management) {
    auto arena_cfg = OrtArenaCfg::Create(...);
    ort_provider_options->UpdateValue("default_memory_arena_cfg", arena_cfg.get());
}  // arena_cfg destroyed here
session_options.AppendExecutionProvider_CUDA_V2(*ort_provider_options);  // dangling pointer

// After: arena_cfg outlives the append call
std::unique_ptr<OrtArenaCfg> arena_cfg;
if (use_arena_management) {
    arena_cfg = OrtArenaCfg::Create(...);
    ort_provider_options->UpdateValue("default_memory_arena_cfg", arena_cfg.get());
}
session_options.AppendExecutionProvider_CUDA_V2(*ort_provider_options);

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