fix: allow adding model names that differ only by case in multi-select

[feitianbubu]

📝 变更描述 / Description

渠道编辑页的模型多选框里，输入 DeepSeek-V3 时如果系统已有小写的 deepseek-v3，"添加自定义模型"选项不会出现，按回车只能选中小写模型，大写模型名无法录入（上游只认大写时渠道就配不了）。

原因是 multi-select.tsx 里判断"输入是否已存在"用了 toLowerCase() 忽略大小写。改为精确匹配后，仅大小写不同的模型名也能正常录入；输入与已有选项完全一致时行为不变。

🚀 变更类型 / Type of change

• 🐛 Bug 修复 (Bug fix) - 请关联对应 Issue，避免将设计取舍、理解偏差或预期不一致直接归类为 bug
• ✨ 新功能 (New feature) - 重大特性建议先通过 Issue 沟通
• ⚡ 性能优化 / 重构 (Refactor)
• 📝 文档更新 (Documentation)

🔗 关联任务 / Related Issue

• Closes 新版UI无法区分大小写，只能强制匹配小写模型 #6029

✅ 提交前检查项 / Checklist

• 人工确认: 我已亲自整理并撰写此描述，没有直接粘贴未经处理的 AI 输出。
• 非重复提交: 我已搜索现有的 Issues 与 PRs，确认不是重复提交。
• Bug fix 说明: 若此 PR 标记为 Bug fix，我已提交或关联对应 Issue，且不会将设计取舍、预期不一致或理解偏差直接归类为 bug。
• 变更理解: 我已理解这些更改的工作原理及可能影响。
• 范围聚焦: 本 PR 未包含任何与当前任务无关的代码改动。
• 本地验证: 已在本地运行并通过测试或手动验证，维护者可以据此复核结果。
• 安全合规: 代码中无敏感凭据，且符合项目代码规范。

📸 运行证明 / Proof of Work

修复前:

修复后:

[coderabbitai]

Walkthrough

The multi-select component now matches typed input against existing option values and labels using case-sensitive comparisons, allowing differently cased entries to remain distinct.

Changes

Multi-select matching

Layer / File(s)	Summary
Exact option matching web/default/src/components/multi-select.tsx	inputMatchesExisting now compares trimmed input directly with option values and labels instead of lowercasing both sides.

Estimated code review effort: 2 (Simple) | ~5 minutes

Poem

A rabbit typed DeepSeek-V3 with care,
No lowercase trap was waiting there.
Exact letters now hold their ground,
Distinct names stay distinct when found.
Hop, hop—matching is fair!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The change matches issue #6029 by allowing model names that differ only by case to be added.
Out of Scope Changes check	✅ Passed	The PR is narrowly scoped to the MultiSelect matching check and its allowCreate behavior.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: multi-select now allows adding names that differ only by case.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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