feat(retrievers): add Keenable web search backend

[ilya-bogin-keenable]

Description

Adds kotaemon.indices.retrievers.keenable_web_search.WebSearch as a third option for KH_WEB_SEARCH_BACKEND, next to the Tavily and Jina retrievers.

The difference from the two existing backends is that it works without an API key. With no KEENABLE_API_KEY set it calls the public endpoint (POST https://api.keenable.ai/v1/search/public); setting a key switches to the authenticated endpoint and only lifts the rate limits. So the @WebSearch chat command can work on a fresh install.

The module follows the shape of tavily_web_search.py:

• same class name and module layout, so the dotted-string setting works unchanged;
• one RetrievedDocument with the results concatenated as URL / title / page text, and the same file_name / type / llm_trulens_score metadata the reasoning pipelines expect;
• max_results, snippet_max_length and timeout exposed as Params;
• a 429 from the API raises a RuntimeError with the retry-after value.

It uses requests, which the package already imports elsewhere; no new dependencies.

Also documented: the new option is listed (commented) in flowsettings.py, .env.example gains a web-search block with the three key variables, and README.md mentions KH_WEB_SEARCH_BACKEND under notable settings. Tavily stays the default.

Tested with libs/kotaemon/tests/test_web_search.py (mocked requests.post: keyless endpoint and headers, keyed endpoint, 429 handling), pre-commit on the changed files, and a live keyless query through the retriever.

Disclosure: I work at Keenable.

Type of change

• New features (non-breaking change).
• Bug fix (non-breaking change).
• Breaking change (fix or feature that would cause existing functionality not to work as expected).

Checklist

• I have performed a self-review of my code.
• I have added thorough tests if it is a core feature.
• There is a reference to the original bug report and related work.
• I have commented on my code, particularly in hard-to-understand areas.
• The feature is well documented.