[jaeminbird]

Summary

_parse_sse_response fails on two standard OpenAI stream shapes, so the HTTP provider check reports a working streaming endpoint as an error:

• A delta with "content": null (OpenAI tool-call chunks, DeepSeek reasoning chunks) reaches "".join(...): sequence item 0: expected str instance, NoneType found.
• With stream_options.include_usage, the last chunk is {"choices": [], "usage": {...}}, and last_data.get("choices", [{}])[0] raises list index out of range.

Content is now appended only when it is a string, and finish_reason comes from the last chunk that carries one.

Type of change

• Bug fix

How was this tested?

• A real local text/event-stream server through _make_http_request: both streams failed before; now success=True with output Hello.
• Two new tests in agent-scan/pytests/test_agent_adapter.py fail on main and pass with the fix; pytest agent-scan/pytests passes (Python 3.12).

Related issue

N/A

Checklist

• go build ./... succeeds locally (no Go changes)
• I followed the existing file/rule format conventions for this contribution type
• I have not introduced hardcoded secrets or credentials

[boy-hack]

Verified locally — both crashes reproduce on main and the fix resolves them cleanly.

Reproduction on main (isolated worktree at origin/main):

• Tool-call/reasoning chunk with "content": null → TypeError: sequence item 0: expected str instance, NoneType found at the "".join(...) accumulation.
• Usage-only final chunk ("choices": [] with stream_options.include_usage) → IndexError: list index out of range at last_data.get("choices", [{}])[0].

Fix verification (isolated worktree at PR head 2d5f8610):

• Null-content delta is skipped (isinstance(content, str) guard); output = "Hello".
• finish_reason is now accumulated from the last chunk that carries one instead of read from the final chunk, so usage-only tails no longer crash; result: finish_reason="stop" with usage preserved.

Suite: main = 68 passed; PR head = 70 passed (the 2 new regression tests account for the delta, zero drift).

CI green (GITHUB_OPENSOURCE_SCAN_PIPELINE@PULL_REQUEST). Diff scope is 2 files in agent-scan only. LGTM — merging.