Add test case coverage for OpenAI backend and CLI frontend

[jbencina]

Summary

This PR creates a mock backend for OpenAI to validate the behavior of all OpenAI API commands used by the application. This mock backend allows for testing of all OpenAI operations (closes #15). The LLM output is not covered by the test suite at this time.

Test coverage added:

• File storage and syncing
• Thread management
• Assistant management
• Chat response

Test Plan

Passed all new unit tests

[Copilot]

Pull Request Overview

This PR adds extensive test coverage for OpenAI backend and CLI frontend functionality through new unit tests and updated settings/CLI output to support recent API changes.

• New tests for file storage/sync, thread, assistant management, and chat response streaming.
• Updated field name from "updated_count" to "remote_count" in sync results and corresponding CLI output.
• Adjusted Settings initialization and client setup to handle a Path-typed settings_path.
• Updated changelog and CI workflow to reflect new test cases and environment variables.

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/openai/test_openai_store.py	Added tests for vector store file management and sync operations
tests/openai/test_openai_chat.py	Added tests covering assistant, thread, and message response
tests/openai/conftest.py	Introduced mocks and fixtures for OpenAI client/backend testing
src/vecsync/store/openai.py	Replaced "updated_count" with "remote_count" in sync results
src/vecsync/settings.py	Updated Settings constructor to accept Path types
src/vecsync/cli/sync.py	Updated CLI output to use the new "remote_count" field
src/vecsync/cli.py	Updated CLI sync command to print "remote_count" consistently
src/vecsync/chat/clients/openai.py	Modified client initialization and settings handling
CHANGELOG.md	Updated changelog with new commands and test case coverage
.github/workflows/ci.yaml	Added dummy OpenAI API key for CI