Add n8n workflow provider with webhook-based execution

[Copilot]

参考 coze, dify provider 以及 n8n 官方文档接口，增加 n8n provider。

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Motivation / 动机

Enable n8n workflow automation integration with AstrBot, allowing users to trigger custom n8n workflows via webhooks and receive responses through the chatbot interface.

Modifications / 改动点

Core Implementation:

• astrbot/core/utils/n8n_api_client.py: Lightweight HTTP client for webhook communication

  • GET/POST methods, streaming/non-streaming responses, SSE support
  • Lazy ClientSession initialization for proper async handling
  • Custom authentication headers

• astrbot/core/provider/sources/n8n_source.py: Provider adapter implementing full Provider interface

  • Configurable input/output key mappings for workflow flexibility
  • Session management, multimodal support (text/images)
  • Intelligent response parsing (string/object/array/media objects)

Integration:

• astrbot/core/provider/manager.py: Provider registration
• dashboard/src/views/ProviderPage.vue: Type mapping
• dashboard/src/utils/providerUtils.js: Provider icon
• tests/test_n8n_provider.py: 13 unit tests covering initialization, methods, parsing

Documentation:

• Updated README.md, README_en.md with n8n provider listing
• Added docs/n8n_provider_implementation.md with configuration guide

Configuration example:

{
  "type": "n8n",
  "id": "my_workflow",
  "enable": true,
  "n8n_webhook_url": "https://n8n.example.com/webhook/abc123",
  "n8n_http_method": "POST",
  "n8n_auth_header": "Authorization",
  "n8n_auth_value": "Bearer token",
  "n8n_output_key": "output",
  "n8n_input_key": "input",
  "timeout": 120,
  "variables": {"key": "value"}
}

Verification Steps / 验证步骤

1. Run tests: uv run pytest tests/test_n8n_provider.py -v
2. Verify import: uv run python -c "from astrbot.core.provider.sources.n8n_source import ProviderN8n"
3. Check provider registration: uv run python -c "from astrbot.core.provider.register import provider_cls_map; print('n8n' in provider_cls_map)"
4. Add n8n provider configuration in WebUI with a test webhook URL
5. Send message to trigger workflow execution

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

================================================= test session starts ==================================================
tests/test_n8n_provider.py::TestN8nProvider::test_provider_registration PASSED                                   [  7%]
tests/test_n8n_provider.py::TestN8nProvider::test_provider_initialization_missing_url PASSED                     [ 15%]
tests/test_n8n_provider.py::TestN8nProvider::test_provider_initialization_invalid_method PASSED                  [ 23%]
tests/test_n8n_provider.py::TestN8nProvider::test_provider_initialization_success PASSED                         [ 30%]
tests/test_n8n_provider.py::TestN8nProvider::test_provider_default_values PASSED                                 [ 38%]
tests/test_n8n_provider.py::TestN8nProvider::test_get_models PASSED                                              [ 46%]
tests/test_n8n_provider.py::TestN8nProvider::test_get_current_key PASSED                                         [ 53%]
tests/test_n8n_provider.py::TestN8nProvider::test_set_key_raises_exception PASSED                                [ 61%]
tests/test_n8n_provider.py::TestN8nProvider::test_forget PASSED                                                  [ 69%]
tests/test_n8n_provider.py::TestN8nProvider::test_terminate PASSED                                               [ 76%]
tests/test_n8n_provider.py::TestN8nProvider::test_parse_n8n_result_string PASSED                                 [ 84%]
tests/test_n8n_provider.py::TestN8nProvider::test_parse_n8n_result_dict_with_output PASSED                       [ 92%]
tests/test_n8n_provider.py::TestN8nProvider::test_parse_n8n_result_dict_without_output_key PASSED                [100%]

================================================== 13 passed in 2.34s ==================================================

CodeQL Security Scan:

Analysis Result for 'python, javascript'. Found 0 alerts:
- **python**: No alerts found.
- **javascript**: No alerts found.

Compatibility & Breaking Changes / 兼容性与破坏性变更

• 这是一个破坏性变更 (Breaking Change)。/ This is a breaking change.
• 这不是一个破坏性变更。/ This is NOT a breaking change.

This is a purely additive change. New provider type with no impact on existing providers or configurations.

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Checklist / 检查清单

• 😊 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。/ If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
• 👀 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"。/ My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
• 🤓 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了 requirements.txt 和 pyproject.toml 文件相应位置。/ 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.
• 😮 我的更改没有引入恶意代码。/ My changes do not introduce malicious code.

Original prompt

参考 coze, dify provider 以及 n8n 官方文档接口，增加n8n provider。

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