fix(Stepper): correct disabled state when min/max are strings

[Mosquito1123]

问题

当 min/max 以字符串传入时（如 min="2" max="10"，这也是本组件 demo 的用法），+ / - 按钮的禁用样式与实际点击逻辑不一致。

<van-stepper min="2" max="10" value="{{ 8 }}" />
- 期望：8 < 10，+ 可用
- 实际：+ 显示为禁用（置灰），但点击又能继续加
fix #6070
根因
packages/stepper/index.ts 的 isDisabled() 用 +value 做数值比较，是正确的：
// plus
return disabled || disablePlus || +currentValue >= +max;
// minus
return disabled || disableMinus || +currentValue <= +min;
但禁用样式是在 packages/stepper/index.wxml 里用裸比较单独算的：
<!-- before -->
disabled: disabled || disablePlus || currentValue >= max
currentValue 恒为字符串；当 min/max 是字符串字面量时，WXML 会走字符串字典序比较："8" >= "10" 为真（比较首字符 "8" > "1"），于是 8 处被误判为已达上限。传数字（min="{{ 2 }}"）时才会走数值比较，所以仅在字符串用法下复现。
修改
- packages/stepper/index.wxml：+ / - 的禁用判断改为数值比较（* 1），与 isDisabled() 语义保持一致
- disabled: disabled || disablePlus || currentValue >= max
+ disabled: disabled || disablePlus || currentValue * 1 >= max * 1
- packages/stepper/demo/index.wxml：新增回归用例 min="2" max="10" value="{{ 8 }}"
- packages/stepper/test/__snapshots__/demo.spec.ts.snap：同步更新快照
验证
- 全量测试通过：55 passed, 56 tests passed
- 回归用例有效性：还原 index.wxml 后，新用例立即失败，快照中 8 处出现 van-stepper__plus--disabled；修复后为 van-stepper__plus（无 --disabled）
- 边界：0、0.5 及数字型 min/max 的行为不变

[Copilot]

🟢 Approval recommended

The implementation consistently matches existing click logic and includes focused regression coverage.

0 open findings

What changed in this PR

Aligns Stepper button styling with its numeric min/max click logic when bounds are strings.

Changes:

• Coerces bounds and current values to numbers in disabled-state comparisons.
• Adds a string-bound demo regression case and updates its snapshot.

File	Description
packages/​stepper/​index.wxml	Fixes numeric disabled-state comparisons.
packages/​stepper/​demo/​index.wxml	Adds the regression example.
packages/​stepper/​test/​__snapshots__/​demo.spec.ts.snap	Verifies correct button styling.

🧠 Review effort: Balanced

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