[Bugfix][Frontend] Handle constrained Harmony output without recipient

[tarjan1]

Purpose

Fixes #45570.

openai-harmony currently parses a standalone constrained output header such
as <|channel|>final <|constrain|>json<|message|>... with
recipient="<|constrain|>json" and content_type=None. The Responses API
therefore routes valid final JSON through the MCP path and returns an
McpCall instead of a ResponseOutputMessage.

This change excludes standalone constrained-format markers from recipient
routing at the Responses output boundary. It covers completed messages,
truncated parser state, streaming deltas, and streaming completion events
without changing the shared Harmony parser.

The underlying parser fix is open as openai/harmony#144.

Duplicate check

This is intentionally narrower than #35906 and #31677:

• [Responses API] Sanitize leaked Harmony control tokens in tool names and recipients #35906 proposes broad sanitization and parser recovery for malformed or
  leaked Harmony control tokens.
• [Bugfix] Sanitize malformed tool call recipients in Harmony parser #31677 sanitizes malformed tool-call recipients.
• This PR only handles a valid standalone constrained-output header that the
  current Harmony dependency misclassifies as a recipient. It does not add a
  parser wrapper or change Chat Completions behavior.

Test Plan

CUDA_VISIBLE_DEVICES=1 .venv-codex/bin/python -m pytest \
  tests/entrypoints/openai/responses/test_harmony_utils.py -q

Run every pre-commit hook applicable to the changed Python files:

.venv-codex/bin/pre-commit run <hook> --files \
  vllm/entrypoints/openai/responses/harmony.py \
  vllm/entrypoints/openai/responses/streaming_events.py \
  tests/entrypoints/openai/responses/test_harmony_utils.py

Test Result

• 34 passed, 17 warnings
• Passed: ruff-check, ruff-format, typos, mypy-3.10,
  check-spdx-header, check-root-lazy-imports, check-filenames,
  check-forbidden-imports, check-torch-cuda-call, validate-config,
  attention-backend-docs, and check-boolean-context-manager.
• The aggregate pre-commit run could not initialize the unrelated
  actionlint environment because its Go download failed with a proxy TLS EOF.
  actionlint does not apply to the changed Python files.

No documentation update is needed because this restores the existing
Responses API message behavior without changing the public API.

AI assistance disclosure: OpenAI Codex assisted with investigation,
implementation, test execution, and drafting this PR. I reviewed every changed
line and validated the behavior and tests.

---

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• The purpose of the PR, including the issue it resolves.
• The test plan and commands.
• The test results.
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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @tarjan1.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork