[Tests] Skip model weight download for render-only test server

[sagearc]

Purpose

RemoteLaunchRenderServer inherits the full model weight download from RemoteVLLMServer.__init__, but the render server only needs the tokenizer — no weights are loaded at runtime.

Changes

• Extract _pre_download_model hook from RemoteVLLMServer.__init__
• Override it as a no-op in RemoteLaunchRenderServer

Test Plan

Existing render tests (test_launch_render.py) cover this path.

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• 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
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[gemini-code-assist]

Code Review

This pull request refactors the model pre-download logic to allow skipping it for the render-only test server, which is a sensible optimization. However, the new implementation for the render server completely skips any pre-download. Since the render server still requires the tokenizer, this could lead to test flakiness if the server startup times out while downloading it. I've provided a suggestion to explicitly pre-download only the tokenizer for the render server to make the tests more robust.

[sagearc]

cc @DarkLight1337 @NickLucche