Add ORT_UNIT_TEST_MAIN_DYNAMIC_PLUGIN_EP_CONFIG_JSON_FILE env var

[edgchen1]

Description

Add support for specifying dynamic plugin EP configuration via a JSON file path in the ORT_UNIT_TEST_MAIN_DYNAMIC_PLUGIN_EP_CONFIG_JSON_FILE environment variable. This is mutually exclusive with the existing inline JSON env var ORT_UNIT_TEST_MAIN_DYNAMIC_PLUGIN_EP_CONFIG_JSON.

Motivation and Context

Allow more flexibility in specifying configuration. It may be impractical to put everything in an environment variable.

[Copilot]

Pull request overview

Adds an alternate way for unit test runs to provide dynamic Plugin EP initialization config by reading JSON from a file path specified in an environment variable, avoiding overly long inline env var values.

Changes:

• Add ORT_UNIT_TEST_MAIN_DYNAMIC_PLUGIN_EP_CONFIG_JSON_FILE env var support to load dynamic plugin EP config JSON from a file.
• Enforce mutual exclusivity between inline JSON (..._CONFIG_JSON) and file-based JSON (..._CONFIG_JSON_FILE).

Comments suppressed due to low confidence (1)

onnxruntime/test/unittest_main/test_main.cc:10

• std::cout/std::cerr are used in this file on non-Windows too, but <iostream> is only included under _WIN32. Please include <iostream> unconditionally (or otherwise ensure the needed declarations are included) to avoid relying on transitive includes that may break on some toolchains/standard libraries.

#include <fstream>
#include <memory>
#include <optional>
#include <string>
#include <vector>

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