[CI Failure] fix test_default_mm_loras

[hl475]

Purpose

LoRA Tests are failing in nightly (https://buildkite.com/vllm/ci/builds/36869/steps/canvas?sid=019a3346-cea1-4bac-ba60-8e4eabb0c2b0, https://buildkite.com/vllm/ci/builds/36869/steps/canvas?sid=019a3346-cea1-4129-9d36-72bb43d662f6, https://buildkite.com/vllm/ci/builds/36869/steps/canvas?sid=019a3346-cea2-4b6b-a02b-b280e7d52b3b) due to recent change from PR - In LongRoPE, decide short vs long based on max_model_len, models with LongRoPE now pick the short‑ or long‑scaling path at init time based on max_model_len. Our LoRA default‑MM tests were using max_model_len=12800, which pushed Phi‑4‑MM into the long‑RoPE path and introduced tiny but deterministic output shifts, , causing brittle exact‑suffix assertions to fail in nightly.

This PR changes max_model_len from 12800 to 4096 to stay on short‑RoPE.

Test Plan

CI

Test Result

https://buildkite.com/vllm/ci/builds/36882/steps/canvas?sid=019a33cd-1ae3-4f0d-a8d5-a939c35eb6e2

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• The test plan, such as providing test command.
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