fix(hunter): bound every tool result; show the provider's reason for an HTTP error

FUXA failed every hunt with HTTP 400, and neither run could say why. The client
discarded the response body. With the body shown, the reason was "This model's
maximum context length...": FUXA ships minified bundles whose single lines run to
megabytes, and grep_repo returned a matched line whole.

- grep_repo and find_refs quote a matched line (240 characters), never the whole
  line.
- Every tool result is capped at 16 x max_chars, with a truncation marker.
- OpenRouterChatClient errors carry the provider's response body (first 500
  characters). That would also have exposed the rejected model id in one call
  instead of eighteen days.

After the fix, FUXA candidates verify normally at $0.004-0.013 each. A test covers
the megabyte-line case.

verify_sinks.py: skip futures cancelled at the spend ceiling instead of reading
them (the run crashed with CancelledError).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

src/openultrasast/hunter_tools.py

@@ -32,6 +32,8 @@ def read_file(root: Path, path: str, *, max_chars: int) -> str:
     return clamped.read_text(errors="ignore")[:max_chars]
 
 
+# A matched line is quoted, not returned whole: a minified bundle's single line can be megabytes.
+_LINE_CHARS = 240
 _Lines = tuple[tuple[str, tuple[str, ...]], ...]
 _SOURCE_MEMO: dict[tuple[str, int], tuple[_Lines, tuple[tuple[Path, int], ...]]] = {}
 
@@ -72,7 +74,7 @@ def grep_repo(root: Path, pattern: str, *, max_matches: int) -> list[dict[str, o
     for relative, lines in _source_lines(root):
         for line_no, line in enumerate(lines, start=1):
             if compiled.search(line):
-                matches.append({"path": relative, "line": line_no, "text": line})
+                matches.append({"path": relative, "line": line_no, "text": line[:_LINE_CHARS]})
                 if len(matches) >= max_matches:
                     return matches
     return matches
@@ -102,7 +104,7 @@ def find_refs(root: Path, symbol: str, mapping_index: object) -> list[dict[str,
         for relative, lines in _source_lines(resolved_root):
             for line_no, line in enumerate(lines, start=1):
                 if compiled.search(line):
-                    refs.append({"path": relative, "name": symbol, "line": line_no, "text": line, "source": "text"})
+                    refs.append({"path": relative, "name": symbol, "line": line_no, "text": line[:_LINE_CHARS], "source": "text"})
     return refs