fix(gateway): reject non-object JSON request bodies

[Tanjiahao (JiahaoTanXX)]

Summary

• reject valid JSON payloads whose top-level value is not an object
• return a clear HTTP 400 response instead of allowing a server-side TypeError
• cover string, array, number, and null payloads with a parameterized regression test

Validation

• pytest -q tests/server/test_endpoints.py (26 passed)
• lightweight CPU test suite (70 passed)
• ruff check .
• ruff format --check .
• python scripts/check_headers.py
• git diff --check

[Copilot]

🟢 Approval recommended

The validation prevents the reported server-side failure and is adequately covered.

Pull request overview

Rejects non-object JSON payloads at the gateway before proxy processing.

Changes:

• Returns HTTP 400 for top-level scalar, array, or null JSON.
• Adds parameterized regression coverage.

File summaries

File	Description
agentlightning/server/routes/proxy.py	Validates that parsed request bodies are objects.
tests/server/test_endpoints.py	Tests rejection of non-object JSON payloads.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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[Tanjiahao (JiahaoTanXX)]

@microsoft-github-policy-service agree