fix: remove All option from KB documents pagination to avoid page_size=-1

[lxfight]

Fixes #9907

In the WebUI knowledge base detail page, switching the documents table pagination to "All" sent page_size=-1 to the backend, where list_documents clamps it with page_size = max(page_size, 1). The response therefore contained only 1 document while total was correct.

The v-data-table-server in DocumentsTab.vue did not set :items-per-page-options, so it inherited Vuetify's default [10, 25, 50, 100, -1], where -1 renders as "All".

This PR follows option A suggested in the issue: explicitly provide [10, 25, 50, 100] (no "All" entry), consistent with other pages such as ConversationPage.vue. The backend clamp is kept as a safety net.

Modifications / 改动点

• dashboard/src/views/knowledge-base/components/DocumentsTab.vue: add :items-per-page-options="[10, 25, 50, 100]" to the documents v-data-table-server, removing the "All" option.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

• npx vue-tsc --noEmit in dashboard/ passes (exit 0).
• Verification steps: open WebUI → Knowledge Base → a KB with ≥ 2 documents → Documents tab → the per-page selector now offers 10 / 25 / 50 / 100 only (no "All"), and every selection returns the expected number of items (page_size echoed correctly).

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Bug Fixes:

• Remove the documents table’s “All” pagination option to prevent requests with page_size=-1 from returning only one document.