resolve(proxy): emit Content-Length: 0 for empty replaced body

build_raw_request's Content-Length guard was "if body and ...", so a
falsy empty-string body (a valid replacement via repeat_request's
modifications["body"]) skipped Content-Length injection entirely.
Sending no Content-Length is valid HTTP/1.1, but some strict servers or
WAFs expect an explicit Content-Length: 0, so silently omitting it can
still cause inconsistent replay behavior.

Change the guard to "body is not None" so an empty body still gets
Content-Length: 0 while a None body (not applicable here, but kept for
clarity) is still skipped.

Adds test_build_raw_request_sets_content_length_zero_for_empty_body,
verified RED against the prior "if body and ..." guard before GREEN.

Addresses review feedback from usestrix/strix#614.

strix/tools/proxy/caido_api.py

@@ -157,7 +157,7 @@ def build_raw_request(
     final_headers = {**headers}
     final_headers.setdefault("Host", parsed.netloc)
     final_headers.setdefault("User-Agent", "strix")
-    if body and "Content-Length" not in {k.title() for k in final_headers}:
+    if body is not None and "Content-Length" not in {k.title() for k in final_headers}:
         final_headers["Content-Length"] = str(len(body.encode("utf-8")))
 
     lines = [f"{method.upper()} {path} HTTP/1.1"]

tests/test_caido_api.py

@@ -60,3 +60,20 @@ def test_build_raw_request_recomputes_content_length_after_replacement() -> None
     )
     head = raw.decode("utf-8").split("\r\n\r\n", 1)[0]
     assert f"Content-Length: {len(new_body.encode('utf-8'))}" in head
+
+
+def test_build_raw_request_sets_content_length_zero_for_empty_body() -> None:
+    components = _components(body="old")
+    modified = apply_modifications(
+        components,
+        {"body": ""},
+        "http://example.com/submit",
+    )
+    _, raw = build_raw_request(
+        method=modified["method"],
+        url=modified["url"],
+        headers=modified["headers"],
+        body=modified["body"],
+    )
+    head = raw.decode("utf-8").split("\r\n\r\n", 1)[0]
+    assert "Content-Length: 0" in head