Fix get_object_details output validation with structured output

[dankelleher]

Summary

• keep get_object_details as a structured-output tool
• return an explicit CallToolResult with structuredContent to avoid injected null optional keys
• preserve text content output while preventing None is not of type 'string' validation failures
• add a regression test for output schema and structured content behavior

Validation

• uv run pytest -q tests/unit/test_tool_schemas.py
• uv run ruff check src/postgres_mcp/server.py tests/unit/test_tool_schemas.py
• MCP Inspector CLI calls for get_object_details on public.Account, public.AccountMember, and public.User against read-only DB creds (all isError=false, no output validation errors)