Fixes for unicode path handling

[jnagi-intel]

Description

Replace path::string() / bare std::filesystem::path(string) with
PathToUTF8String / ToPathString in two places that handle user-supplied
paths.

Motivation and Context

On Windows, path::string() and std::filesystem::path(std::string) use the
system ANSI code page (CP_ACP). When a model or EPContext output file sits in a
directory with non-ASCII Unicode characters, this corrupts the path and causes:

• ModelMetadefIdGenerator::GenerateId — throws during session initialization
  ("No mapping for the Unicode character exists in the target multi-byte code page")
• ModelGenOptions (EPContext options) — constructs a garbled path, failing
  EPContext file creation with ENOENT

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates EP context output path handling to correctly interpret UTF-8 session config values as Unicode paths on Windows.

Changes:

• Add ToPathString() conversion when constructing std::filesystem::path from ep_context_file_path.
• Add explanatory comments about Windows UTF-8 vs ANSI code page path decoding.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[adrianlizarraga]

Would be great to add a unit test that uses non-ascii unicode characters for model compilation. Could be placed in this file:

onnxruntime/onnxruntime/test/autoep/test_execution.cc

Line 778 in a49da87

TEST(OrtEpLibrary, PluginEp_GenEpContextModel_ErrorOutputModelExists_AutoGenOutputModelName) {

[jnagi-intel]

Would be great to add a unit test that uses non-ascii unicode characters for model compilation. Could be placed in this file:

onnxruntime/onnxruntime/test/autoep/test_execution.cc

Line 778 in a49da87

TEST(OrtEpLibrary, PluginEp_GenEpContextModel_ErrorOutputModelExists_AutoGenOutputModelName) {

@adrianlizarraga Added tests for both the fixes.

[adrianlizarraga]

One more small issue.