refactor: rename vl package to multimodal

[CUHKSZzxy]

Summary

• Rename the top-level multimodal implementation package from lmdeploy.vl to lmdeploy.multimodal.
• Update internal imports, docs, and tests to use lmdeploy.multimodal.
• Leave lmdeploy.vl as a top-level migration error instead of keeping backward-compatible deep imports.
• Rename the focused test directory from tests/test_lmdeploy/test_vl to tests/test_lmdeploy/test_multimodal.

Prerequisite

This PR is stacked on top of InternLM#4693 and should be reviewed/merged after that prerequisite cleanup.

Validation

• Syntax checks for the moved lmdeploy.multimodal package and the top-level lmdeploy.vl migration error.
• Import smoke for canonical lmdeploy.multimodal APIs and the expected lmdeploy.vl failure.
• Focused multimodal pytest coverage for preprocessing utilities, safe URL handling, Qwen3-Omni processor behavior, Qwen3-VL processor behavior, and media encode/decode helpers.
• Diff whitespace check.

Assistance

Assisted with Codex + GPT-5.5 xHigh Fast, reviewed manually