feat(kb): support pluggable knowledge base retrieval backends

[lxfight]

AstrBot plugins can already integrate external knowledge base systems through custom hooks, but there is no stable, shared contract for knowledge base discovery and retrieval.

This PR introduces a minimal public backend API so plugins can expose external knowledge bases through a consistent interface without depending on AstrBot's built-in storage implementation. The scope is intentionally limited to enabled knowledge base discovery and read-only retrieval.

Modifications / 改动点

• Added public plugin contracts for knowledge base backends:

  • BaseKnowledgeBaseBackend
  • KnowledgeBaseRef
  • KnowledgeBaseInfo
  • KnowledgeBaseQuery
  • KnowledgeBaseHit
  • KnowledgeBaseResponse
  • Standard backend error types

• Added backend registration and unregistration through the plugin Context.

• Added an adapter that exposes the built-in knowledge base through the same contract without replacing the existing built-in Agent flow.

• Added concurrent multi-backend retrieval with:

  • Per-backend timeouts
  • Failure isolation
  • Rank-based result merging
  • Global top_k truncation
  • Runtime validation of third-party responses

• Added first-class knowledge base identity to every retrieval hit.

• Integrated enabled external backends into the existing Agent knowledge base flow.

• Defined list_knowledge_bases() as returning only knowledge bases that are enabled and accessible for the current session.

• Added explicit plugin reload lifecycle behavior. Plugins unregister their backends in terminate() before releasing resources.

• Added Chinese and English plugin development documentation with a complete backend example.

• Added unit and adversarial tests for registration, discovery, retrieval, failure isolation, malformed responses, result identity, and reload behavior.

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

Scope

This PR intentionally does not standardize:

• Knowledge base creation or deletion
• Document upload or management
• Chunk or index management
• Backup and restore
• Credential configuration
• Unified WebUI management

Plugins may provide those capabilities through their own configuration, commands, or Plugin Pages. Optional management contracts can be considered separately when concrete cross-backend requirements are available.

The existing AstrBot knowledge base database, upload flow, Dashboard APIs, and backup behavior are unchanged.

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

Verification performed:

uv run ruff format .
503 files left unchanged

uv run ruff check .
All checks passed

Knowledge base test suite:
51 passed

Unit test suite:
906 passed

Two existing Dashboard log-capture tests fail locally because warnings are emitted through the configured logger instead of caplog. The same failures were reproduced on upstream/master and are unrelated to this PR.

The Chinese and English VitePress documentation build also completed successfully.

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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

Enable plugins to provide external knowledge bases through a shared, resilient retrieval backend contract.

New Features:

• Add a public plugin API for discovering and performing read-only retrieval from external knowledge bases.
• Integrate enabled external backend results into the Agent knowledge base context alongside built-in results.

Enhancements:

• Provide concurrent multi-backend retrieval with timeouts, failure isolation, rank-based merging, global result limits, and response validation.
• Expose the built-in knowledge base through the shared backend contract while preserving its existing retrieval flow.
• Add backend lifecycle registration through plugin context and preserve backend identity across discovery and retrieval results.

Documentation:

• Add English and Chinese plugin documentation with a complete external knowledge base backend example and API usage guidance.

Tests:

• Add unit and adversarial coverage for backend contracts, registration, discovery, retrieval orchestration, malformed responses, failure isolation, result identity, built-in adaptation, and plugin reload behavior.