fix(anthropic_sdk): preserve empty tool output + harden token test

Two more reviewer-flagged issues:

1. _call_tool_result_to_text() dropped empty TextContent

   The truthy check 'if text:' treated TextContent(text='') the same as
   text=None and skipped it. With an only-empty content list, parts
   would be [] and the helper fell through to the str(result) fallback
   (which is a noisy repr of the result object) instead of returning
   the actual empty result the tool reported. Fix: 'if text is not
   None:' preserves explicit empty strings; the str(result) fallback
   now only fires when there are no text-bearing blocks at all.

2. test_build_raises_bad_request_when_no_token_available was flaky

   The test cleared a long list of API key env vars (defensive cargo
   cult) but missed COPILOT_TOKEN, which is the second variable that
   capi.get_AI_token() consults. On runners with COPILOT_TOKEN set
   (e.g. CI envs authed to Copilot), the test would unexpectedly find
   a token and the assertion would fail. Simplified to clear only the
   two vars the chain actually consults: AI_API_TOKEN and COPILOT_TOKEN.

+2 regression tests for empty-string preservation; 291 passing.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: anticomputer

src/seclab_taskflow_agent/sdk/anthropic_sdk/backend.py

@@ -54,12 +54,18 @@ def _mcp_tools_to_anthropic(tools: list[Any]) -> list[dict[str, Any]]:
 
 
 def _call_tool_result_to_text(result: Any) -> str:
-    """Extract text from an MCP CallToolResult."""
+    """Extract text from an MCP CallToolResult.
+
+    Preserves empty strings: a tool that returns ``TextContent(text="")``
+    is returning an explicit empty result, not "no content".  Only fall
+    back to ``str(result)`` (a noisy repr) when there are genuinely no
+    text-bearing content blocks at all.
+    """
     content = getattr(result, "content", [])
     parts = []
     for c in content:
         text = getattr(c, "text", None)
-        if text:
+        if text is not None:
             parts.append(text)
     return "\n".join(parts) if parts else str(result)
 

tests/test_sdk_anthropic_adapter.py

@@ -193,6 +193,25 @@ def test_call_tool_result_to_text_empty():
     assert isinstance(text, str)
 
 
+def test_call_tool_result_to_text_preserves_empty_string():
+    """A tool returning TextContent(text='') is reporting an explicit
+    empty result. The helper must return '' verbatim, not fall back to
+    str(result) (which is a noisy repr of the result object).
+
+    Regression for the truthy-check bug: ``if text:`` was treating ''
+    the same as None and dropping it, causing the empty content list
+    branch to fire and emit ``str(result)`` to the model.
+    """
+    result = type("R", (), {"content": [_FakeContent("")]})()
+    assert _call_tool_result_to_text(result) == ""
+
+
+def test_call_tool_result_to_text_preserves_empty_among_nonempty():
+    """Empty TextContent should join with neighbors as ''."""
+    result = type("R", (), {"content": [_FakeContent("a"), _FakeContent(""), _FakeContent("b")]})()
+    assert _call_tool_result_to_text(result) == "a\n\nb"
+
+
 # -- bearer_auth via provider registry --
 
 
@@ -480,13 +499,18 @@ def test_build_raises_bad_request_when_no_token_available(monkeypatch):
     Otherwise the Anthropic client gets created with an empty 'Bearer '
     header and the failure surfaces later as an opaque 401 mid-stream
     instead of a clear BackendBadRequestError at build time.
+
+    Clears every variable consulted by ``capi.get_AI_token``
+    (``AI_API_TOKEN`` then ``COPILOT_TOKEN``) to keep the test
+    deterministic regardless of the runner's ambient environment.
     """
     import asyncio
 
-    # Clear every token-source env var the standard chain consults
-    for var in ("AI_API_TOKEN", "OPENAI_API_KEY", "AZURE_OPENAI_API_KEY",
-                "ANTHROPIC_API_KEY", "GITHUB_TOKEN", "GH_TOKEN"):
-        monkeypatch.delenv(var, raising=False)
+    # Must clear *every* env var the token chain consults; missing
+    # COPILOT_TOKEN here would make the test flaky on runners that
+    # happen to have it set (e.g. CI machines authed to copilot).
+    monkeypatch.delenv("AI_API_TOKEN", raising=False)
+    monkeypatch.delenv("COPILOT_TOKEN", raising=False)
 
     spec = AgentSpec(
         name="t",