webgpu: fix RecurrentState graph capture with shared buffer aliasing

[qjia7]

Summary

• RecurrentState::share_buffers_ now derives from past_present_share_buffer config (same pattern as DefaultKeyValueCache) instead of being hardcoded to false for WebGPU
• Add a graph-capture guard that throws when share_buffers_ is false and graph capture is requested, matching the existing guard in kv_cache.cpp
• Add Ort::Experimental::Get_*_Fn accessor shim to onnxruntime_api.h so genai builds against ORT builds where onnxruntime_experimental_cxx_api.h cannot be included directly

Motivation

WebGPU graph capture freezes GPU buffer handles at the first captured decode step. RecurrentState was using separate past/present buffers for WebGPU with a per-step C++ pointer swap — but graph capture ignores those swaps on replay, so every decode step after the first read a stale buffer, producing garbage output.

With past_present_share_buffer: true, the ORT LinearAttention and CausalConvWithState kernels detect the aliased buffers via initial_state_in_present_state / conv_state_in_present_state and switch to a single read_write binding, which satisfies the WebGPU spec constraint that originally motivated the per-device split. Buffer handles stay stable across graph capture replays.

The original comment in the code noted a TODO to remove the WebGPU special case once the ORT WebGPU EP kernels natively supported past/present buffer sharing — that support is already present, so this PR resolves the TODO.

Test plan

• Qwen3.5-0.8B-webgpu-fused correctness test: all queries pass with enableGraphCapture=1
• Qwen3.5-0.8B-webgpu-fused multi-gen test: sequential and overlapping generators pass with enableGraphCapture=1
• Both tests also pass with enableGraphCapture=0 (no regression on non-graph-capture path)
• clang-format: no issues on changed files
• Build: succeeds against local ORT WebGPU EP build

[Copilot]

Pull request overview

This PR fixes incorrect WebGPU graph-capture behavior for models that use RecurrentState by making recurrent past/present buffer sharing follow the past_present_share_buffer configuration (so GPU buffer handles remain stable across graph-capture replays). It also adds a small ONNX Runtime experimental-API accessor shim so GenAI can build in environments where the experimental C++ header cannot be included.

Changes:

• Derive RecurrentState::share_buffers_ from GeneratorParams::IsPastPresentShareBufferEnabled(...) (instead of a WebGPU hardcoded behavior).
• Add a runtime guard that rejects graph capture when effective past/present sharing is disabled.
• Add Ort::Experimental::Get_*_Fn accessor shims in onnxruntime_api.h to avoid including onnxruntime_experimental_cxx_api.h.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/models/recurrent_state.h	Updates the member comment to describe config-driven past/present sharing semantics.
src/models/recurrent_state.cpp	Switches share_buffers_ to be config-derived and adds a graph-capture precondition check.
src/models/onnxruntime_api.h	Adds an experimental accessor shim to support model-package functions without including ORT experimental C++ headers.