fix(responses): stop marking native collaboration calls as plaintext

[HouMinXi]

Summary

• encrypted_function_args set to an empty array is the marker Codex uses for a call delivered as plaintext. The non-streaming translator stamped that marker onto every collaboration call, including ones that arrived already in Responses shape.
• The sanitizer rebuilt each function_call without its namespace, status or encrypted arguments, and the stream path restored the identity without writing it back into the forwarded event.
• Only a request translated from Chat into Responses gets the empty marker now. A native Responses call keeps the upstream fields as they arrived.

Related Issues

• Closes fix(api): native Responses collaboration calls falsely marked plaintext #15088

Validation

• Change type: other (Responses passthrough)
• Focused tests: node --import tsx/esm --test tests/unit/issue-15088-native-responses-collaboration-plaintext.test.ts (6 pass)
• eslint on the changed files, with the repo suppression file
• Reconciled with release/v3.8.52 @ 23a1148486
• New test covers the change
• Defect injection: restoring the unconditional marker makes the native passthrough case fail, and the change was restored

Tests Added Or Updated

• tests/unit/issue-15088-native-responses-collaboration-plaintext.test.ts (added)

Coverage Notes

• The test asserts a native collaboration call keeps its namespace and does not gain an empty encrypted_function_args, while a call translated from Chat still gets the marker.

Reviewer Notes

• The Chat-to-Responses path still stamps the empty marker. That is the correct signal for a call OmniRoute translated from plaintext, and the existing cases cover it.