fix shape inference error for ep context nodes (#25398)
### Description
To support writing an Execution Provider (EP) using the new EP ABI introduced in #24887, this PR adds value info for EP Context nodes to prevent shape inference errors during `Graph::Resolve`.
### Motivation and Context
When creating a new EP Context node whose input is the output of another EP Context node, Graph::Resolve fails to set the type for the new node's arguments. This is because EP Context nodes do not have a TypeAndShapeInferenceFunction defined, as shown here:
https://github.com/microsoft/onnxruntime/blob/5fdd4e4f2a2b6705a9a49a378a3b3496805067ee/onnxruntime/core/graph/contrib_ops/contrib_defs.cc#L3289-L3337
As a result, an exception is thrown during shape inference:
https://github.com/microsoft/onnxruntime/blob/5fdd4e4f2a2b6705a9a49a378a3b3496805067ee/onnxruntime/core/graph/graph.cc#L2964
Specifically:
EP Context nodes lack TypeAndShapeInferenceFunction, so onnx_inferred_type is unavailable. existing_type is nullptr due to the logic in:
https://github.com/microsoft/onnxruntime/blob/9de58ac7a3d18d6ae7f7ae502b3f91361067f1b5/onnxruntime/core/session/ep_plugin_provider_interfaces.cc#L279
https://github.com/microsoft/onnxruntime/blob/9de58ac7a3d18d6ae7f7ae502b3f91361067f1b5/onnxruntime/core/session/ep_plugin_provider_interfaces.cc#L285
### Implementation
This PR attempts to add type information to EP Context nodes with best effort, ensuring that Graph::Resolve can proceed without errors even when type inference is not explicitly defined.
