[Chore] Deprecate SupportsMultiModal.merge_by_field_config

[DarkLight1337]

Purpose

Follow-up to #30035

Since merge_by_field_config is no longer effective, we can remove it from all model classes early and emit errors/warning messages for OOT models that still define merge_by_field_config to help them migrate.

Test Plan

Test Result

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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.

[gemini-code-assist]

Code Review

This pull request effectively deprecates the merge_by_field_config attribute from the SupportsMultiModal protocol. The changes involve removing this redundant flag from a large number of multi-modal model classes, which simplifies the codebase. The core logic change in vllm/model_executor/models/interfaces.py correctly implements the deprecation by raising an error for the now-unsupported False value and issuing a warning for the redundant True value, providing clear guidance for developers. This is a good cleanup effort that improves maintainability.