Check /reranker `top_n` at the REST level

[mkhludnev]

Is your feature request related to a problem? Please describe.
While working on #7189, #6700 esp. #7199, I realized that there's no constraint check for top_n parameter at the REST level.

Describe the solution you'd like

Providing https://api.jina.ai/redoc#tag/rerank/operation/rank_v1_rerank_post and https://jina.ai/reranker/ as a spec

• top_n<=0 - respond with 422, see below
• top_n>=len(documents) or top_n is absent ;-( although I've got 500 from JINA API when omit it) - just ignore it, respond with score desc docs
• 0<top_n<len(documents) - crop score desc docs

"detail": "[] Validation error: 'body -> jina-reranker-v3 -> top_n' Input should be greater than or equal to 1. Field errors: : Invalid value",
  "errors": [
    {
      "field": "body -> jina-reranker-v3 -> top_n",
      "message": "Input should be greater than or equal to 1",
      "type": "greater_than_equal",
      "input": "0"
    }
  ]

I suppose preliminary check will simplify backends, reducing a need in constraint check.