Add extra handling for invalid ORT format model.

[skottmckay]

Description

Improve handling for invalid ORT format model.

Motivation and Context

MSRC 119613

[Copilot]

Pull request overview

This PR hardens ORT-format model loading against semantically corrupt FlatBuffers content by converting previously-crashing null dereferences/assertions into clean Status failures with actionable error messages. It also adds targeted regression tests that construct minimal ORT-format FlatBuffers by hand to validate the new rejection paths.

Changes:

• Add validation in Graph::LoadFromOrtFormat to reject NodeEdge entries that reference an unpopulated node slot.
• Add validation in Node::LoadEdgesFromOrtFormat to reject EdgeEnd references to missing nodes.
• Add validation for graph inputs/outputs referencing unknown NodeArg names, and add regression tests for all three cases.

Reviewed changes

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

File	Description
onnxruntime/test/ir/graph_test.cc	Adds regression tests that build minimal ORT-format FlatBuffers to ensure invalid graphs fail with clear errors instead of crashing.
onnxruntime/core/graph/graph.cc	Adds defensive null checks and clearer error returns for corrupt ORT-format edges and graph IO NodeArg references.

[skottmckay]

This overlaps with #28186. @tianleiwu is there anything from this PR worth keeping? Maybe the tests?

[tianleiwu]

@skottmckay Thanks! I've consolidated the unique test coverage from this PR into #28186 (commit 23f9aa3).

Comparing your three tests against the existing tests in 28186:

• RejectsNodeEdgeForUnpopulatedNodeSlot → already covered by RejectsDanglingNodeEdge
• RejectsEdgeEndForUnpopulatedNodeSlot → only the out-of-range path was covered; I added RejectsEdgeEndReferencingNullNodeSlot for the within-range null-slot case
• RejectsGraphIoWithUnknownNodeArgName → was not covered; added as RejectsGraphInputWithUnknownNodeArg

The graph.cc hardening overlaps with what 28186 already does, so I kept 28186's implementation and just folded in the missing test coverage. This PR can be closed in favor of #28186.