[WIP] Align with upstream vLLM v0.15.0 pre-release

[Copilot]

• Understand the task: Generate release notes for v0.16.0rc1 following v0.15.0rc1 format
• Fetch and analyze all commits between d6f93b0 and 75770c9 (82 commits total)
• Categorize commits into appropriate sections (Features, Bugfixes, CI/Docs, etc.)
• Identify new contributors from commit history
• Create structured release notes following the v0.15.0rc1 template
• Save release notes to appropriate file or output format

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