[grpc] Unify ResponsesContext and HarmonyResponsesContext

[CatherineSue]

Motivation

ResponsesContext only differs with HarmonyResponsesContext in backgroud_tasks, which is not actually used anywhere. cancel_response depends on it but it will fail at the first step.

Modifications

• Move ResponsesContext to common/context.rs
• Remove backgroud_tasks in ResponsesContext
• Remove HarmonyResponsesContext
• Update cancel_response_impl logic to indicate it is not supported.

Accuracy Tests

Benchmarking and Profiling

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.

Review Process

1. Ping Merge Oncalls to start the PR flow. See the PR Merge Process.
2. Get approvals from CODEOWNERS and other reviewers.
3. Trigger CI tests with comments (/tag-run-ci-label, /rerun-failed-ci, /tag-and-rerun-ci) or contact authorized users to do so.
4. After green CI and required approvals, ask Merge Oncalls to merge.

[gemini-code-assist]

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