Isolate MCP OAuth settings persistence in agent-server tests

The MCP OAuth settings-store tests should keep persistence inside their pytest temporary directory by setting OH_PERSISTENCE_DIR.

Actual Behavior

In tests/agent_server/test_mcp_oauth_store.py, the affected tests create configurations using tmp_path but call get_settings_store() without setting OH_PERSISTENCE_DIR. The settings store therefore falls back to the user default ~/.openhands persistence directory, allowing encrypted MCP OAuth state to be written outside the pytest sandbox.

Reproduce with:

uv run pytest tests/agent_server/test_mcp_oauth_store.py -q

Before this change, the tests did not redirect OH_PERSISTENCE_DIR to tmp_path.

Acceptance Criteria

• Each affected MCP OAuth settings-store test sets OH_PERSISTENCE_DIR to its pytest tmp_path.
• The focused test suite passes with uv run pytest tests/agent_server/test_mcp_oauth_store.py -q.
• Test execution does not write settings or encrypted OAuth state to the user default persistence directory.

This issue was created and updated by an AI agent (OpenHands) on behalf of the user.

Start in tests/agent_server/test_mcp_oauth_store.py and run uv run pytest tests/agent_server/test_mcp_oauth_store.py -q. Inspect each affected test that uses tmp_path and get_settings_store(), then ensure OH_PERSISTENCE_DIR points to that temporary directory. Done means the focused suite passes and no settings or encrypted OAuth state is written under the user default persistence directory.