Feat/searxng

[Andres77872]

### Description
This pull request introduces SearXNG metasearch provider integration into the web search module, along with supporting configuration changes. Key updates include:
- Addition of `base_url` configuration for SearXNG.
- Unified search provider settings across modules.
- Integration of templates, context handling, and API support for SearXNG.
- Update of API key handling to support masking and better preserve existing keys in forms.

### Related Issues
- Closes #...
- Related to #...

### Module(s) Affected
- [ ] `agents`
- [x] `api`
- [x] `config`
- [ ] `core`
- [ ] `knowledge`
- [ ] `logging`
- [x] `services`
- [ ] `tools`
- [ ] `utils`
- [x] `web` (Frontend)
- [ ] `docs` (Documentation)
- [ ] `scripts`
- [ ] `tests`
- [ ] Other: `...`

### Checklist
- [ ] I have read and followed the [contribution guidelines](https://github.com/HKUDS/DeepTutor/blob/dev/CONTRIBUTING.md).
- [ ] My code follows the project's coding standards.
- [ ] I have run `pre-commit run --all-files` and fixed any issues.
- [ ] I have added relevant tests for my changes.
- [ ] I have updated the documentation (if necessary).
- [ ] My changes do not introduce any new security vulnerabilities.

### Additional Notes
This PR also includes cleanup of prior SearXNG-related changes that were deemed unnecessary and merges the latest updates from `main`.