[CI/Build] Remove use of skip_v1

[DarkLight1337]

Purpose

CI can pass regardless of skip_v1 flag (except for https://buildkite.com/vllm/ci/builds/51898/steps/canvas?sid=019c6b08-b21c-4c5e-9194-91966b03d9b4&tab=output uncovered by #34560, which will be handled in a separate PR) so let's remove them from all tests.

Also fixed detokenizer tests not actually being run in CI.

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 removes the skip_v1 pytest marker and its usage across the codebase. The changes are consistent and correctly remove the marker from CI configurations, the pytest configuration in pyproject.toml, and a test case. The changes are straightforward, correct, and align with the stated purpose of the PR. I have no issues to report.