[Disagg] Support large batch size in proxy server and update NixlConnector doc for DP

[minosfuture]

Purpose

This is to fix the http.ReadError when concurrency (batch size) is high, by

• allow unlimited connection
• close connection asap

Also include minor update to the NixlConnector usage doc for DP deployment.

Test Plan

p/d disagg with both proxy script with concurrency = 4096.

Test Result

No more request failure.

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

Documentation preview: https://vllm--28782.org.readthedocs.build/en/28782/

[gemini-code-assist]

Code Review

This pull request addresses http.ReadError in proxy servers under high concurrency. The fix involves two main changes to the httpx.AsyncClient usage: allowing unlimited connections by adjusting limits, and ensuring connections are released promptly by consuming the response body with await response.aread(). These changes are consistently applied to both disagg_proxy_server.py and toy_proxy_server.py. Additionally, a helpful clarification has been added to the NixlConnector documentation regarding multi-host DP deployment. The changes are logical, well-targeted, and effectively resolve the stated issue. The code quality is good, and I have no further suggestions.