Improve error message when EPContext node is not assigned to an EP

[adrianlizarraga]

Description

Improves the error message returned by ORT when loading a compiled model in a session that does not have the required execution provider(s).

For example, ORT now returns the following error when creating session with a model compiled explicitly for OpenVINO EP without adding OpenVINO EP to the session:

EPContext node generated by 'OpenVINOExecutionProvider' is not compatible with any execution provider added to the session. EPContext node name: 'EPContextNode0'. Available session execution providers: [CPUExecutionProvider].

Compare the above message with the more generic message that is currently returned:

Could not find an implementation for EPContext(1) node with name 'EPContextNode0'

Motivation and Context

Improves diagnosability when loading of pre-compiled models fails. Specifically, the ambiguity of the original message led to many hours spent debugging an error where a compiled model failed to run because the expected OrtEpDevice was inadvertently not added to a session.

[Copilot]

Pull request overview

Improves diagnosability when loading pre-compiled models containing EPContext nodes by emitting a targeted error if an EPContext node is not assigned to any execution provider, and adds a regression test to validate the new message.

Changes:

• Add a specialized NOT_IMPLEMENTED error message for unassigned EPContext nodes that includes the node’s source attribute and the session’s registered EPs.
• Add a new AutoEP test that compiles a model with the example plugin EP and verifies the explicit error when loading it without registering the required EP.

Reviewed changes

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

File	Description
onnxruntime/test/autoep/test_execution.cc	Adds regression test covering the explicit error when loading an EPContext model without the required EP.
onnxruntime/core/framework/session_state.cc	Enhances node-to-EP assignment verification to emit a descriptive error for unassigned EPContext nodes.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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