chore: 添加虚拟环境到仓库

- 添加 backend_service/venv 虚拟环境
- 包含所有Python依赖包
- 注意：虚拟环境约393MB，包含12655个文件

backend_service/venv/lib/python3.13/site-packages/sympy/series/series.py

@@ -0,0 +1,63 @@
+from sympy.core.sympify import sympify
+
+
+def series(expr, x=None, x0=0, n=6, dir="+"):
+    """Series expansion of expr around point `x = x0`.
+
+    Parameters
+    ==========
+
+    expr : Expression
+           The expression whose series is to be expanded.
+
+    x : Symbol
+        It is the variable of the expression to be calculated.
+
+    x0 : Value
+         The value around which ``x`` is calculated. Can be any value
+         from ``-oo`` to ``oo``.
+
+    n : Value
+        The number of terms upto which the series is to be expanded.
+
+    dir : String, optional
+          The series-expansion can be bi-directional. If ``dir="+"``,
+          then (x->x0+). If ``dir="-"``, then (x->x0-). For infinite
+          ``x0`` (``oo`` or ``-oo``), the ``dir`` argument is determined
+          from the direction of the infinity (i.e., ``dir="-"`` for
+          ``oo``).
+
+    Examples
+    ========
+
+    >>> from sympy import series, tan, oo
+    >>> from sympy.abc import x
+    >>> f = tan(x)
+    >>> series(f, x, 2, 6, "+")
+    tan(2) + (1 + tan(2)**2)*(x - 2) + (x - 2)**2*(tan(2)**3 + tan(2)) +
+    (x - 2)**3*(1/3 + 4*tan(2)**2/3 + tan(2)**4) + (x - 2)**4*(tan(2)**5 +
+    5*tan(2)**3/3 + 2*tan(2)/3) + (x - 2)**5*(2/15 + 17*tan(2)**2/15 +
+    2*tan(2)**4 + tan(2)**6) + O((x - 2)**6, (x, 2))
+
+    >>> series(f, x, 2, 3, "-")
+    tan(2) + (2 - x)*(-tan(2)**2 - 1) + (2 - x)**2*(tan(2)**3 + tan(2))
+    + O((x - 2)**3, (x, 2))
+
+    >>> series(f, x, 2, oo, "+")
+    Traceback (most recent call last):
+    ...
+    TypeError: 'Infinity' object cannot be interpreted as an integer
+
+    Returns
+    =======
+
+    Expr
+        Series expansion of the expression about x0
+
+    See Also
+    ========
+
+    sympy.core.expr.Expr.series: See the docstring of Expr.series() for complete details of this wrapper.
+    """
+    expr = sympify(expr)
+    return expr.series(x, x0, n, dir)