Add test matrix for multiple Fess/OpenSearch versions and dynamic Docker Compose selection

[marevol]

This PR introduces support for testing against multiple versions of Fess and OpenSearch by:

• Updating the GitHub Actions workflow:
  • Renamed job from build to test.
  • Added a matrix strategy to run tests with:
    • Fess 14.19.2 + OpenSearch 2.19.1
    • Fess 15.1.0 + OpenSearch 3.1.0
  • Exposed FESS_VERSION and OPENSEARCH_VERSION as environment variables for test runs.
• Updating tests/conftest.py:
  • The Docker Compose file is now dynamically selected based on the FESS_VERSION environment variable.
  • Uses compose-fess15.yaml for Fess 15.x, otherwise defaults to compose-fess14.yaml.

This ensures broader compatibility testing and aligns test infrastructure with multiple Fess versions.

[Copilot]

Pull Request Overview

This PR enhances the testing infrastructure to support multiple Fess and OpenSearch version combinations by implementing a matrix strategy in GitHub Actions and dynamic Docker Compose file selection. The change enables testing against both legacy (14.19.2/2.19.1) and current (15.1.0/3.1.0) versions to ensure broader compatibility.

• Renamed GitHub Actions job from "build" to "test" and added matrix strategy for two version combinations
• Implemented dynamic Docker Compose file selection based on FESS_VERSION environment variable
• Created separate compose files for Fess 14.x and 15.x with appropriate version defaults

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
.github/workflows/run-tests.yml	Added matrix strategy for testing multiple Fess/OpenSearch version combinations
tests/conftest.py	Implemented dynamic compose file selection logic based on FESS_VERSION
tests/compose-fess15.yaml	Updated existing compose file to use environment variables for Fess 15.x
tests/compose-fess14.yaml	New compose file created for Fess 14.x compatibility testing

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