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.

Prerequisite

Depends on #4693. This PR should be reviewed after that prerequisite cleanup.

Validation

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

Assistance

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