resolve(proxy): recompute Content-Length when repeat_request replaces body

Problem:
apply_modifications() let modifications={"body": ...} replace the request
body but left the original Content-Length header untouched.
build_raw_request() only fills in Content-Length when the header is
absent, so the stale, smaller value from the original request was sent
on the wire alongside the new, larger body.

Impact:
A server reading exactly Content-Length bytes truncates the replayed
body (or stalls waiting for more), silently discarding the
modified/injected payload. This defeats parameter-tampering and
injection replays, which is the core use case of repeat_request, and
produces misleading negative results during testing.

Resolution:
When modifications["body"] is present, drop any existing Content-Length
header from the carried-over headers (matched case-insensitively),
unless the caller explicitly supplies Content-Length in this same call's
modifications["headers"]. This lets build_raw_request's existing
"fill in if absent" logic recompute the correct length from the new
body.

Testing:
Added tests/test_caido_api.py covering: stale header dropped on body
replacement, explicit caller-supplied Content-Length respected, headers
left untouched when body isn't modified, and an end-to-end check that
build_raw_request emits the recomputed length in the raw HTTP request.
Verified RED (failing on pre-fix code, reproducing the exact stale
Content-Length: 3 scenario from the bug report) before GREEN.

Resolves #603

strix/tools/proxy/caido_api.py

@@ -261,6 +261,12 @@ def apply_modifications(
         headers.update(modifications["headers"])
     if "body" in modifications:
         body = modifications["body"]
+        explicit_content_length = "Content-Length" in {
+            k.title() for k in modifications.get("headers", {})
+        }
+        if not explicit_content_length:
+            for key in [k for k in headers if k.title() == "Content-Length"]:
+                del headers[key]
     if "cookies" in modifications:
         cookies: dict[str, str] = {}
         if headers.get("Cookie"):

tests/test_caido_api.py

@@ -0,0 +1,62 @@
+"""Tests for request modification in the Caido proxy integration."""
+
+from __future__ import annotations
+
+from strix.tools.proxy.caido_api import apply_modifications, build_raw_request
+
+
+def _components(body: str = "old", headers: dict[str, str] | None = None) -> dict:
+    return {
+        "method": "POST",
+        "url_path": "/submit",
+        "headers": headers or {"Content-Length": str(len(body))},
+        "body": body,
+    }
+
+
+def test_apply_modifications_drops_stale_content_length_on_body_replace() -> None:
+    components = _components(body="old")
+    result = apply_modifications(
+        components,
+        {"body": "a much longer replacement body than the original"},
+        "http://example.com/submit",
+    )
+    assert "Content-Length" not in {k.title() for k in result["headers"]}
+
+
+def test_apply_modifications_keeps_explicit_content_length_on_body_replace() -> None:
+    components = _components(body="old")
+    result = apply_modifications(
+        components,
+        {"body": "new-body", "headers": {"Content-Length": "8"}},
+        "http://example.com/submit",
+    )
+    assert result["headers"]["Content-Length"] == "8"
+
+
+def test_apply_modifications_without_body_change_keeps_headers() -> None:
+    components = _components(body="old")
+    result = apply_modifications(
+        components,
+        {"headers": {"X-Test": "1"}},
+        "http://example.com/submit",
+    )
+    assert result["headers"]["Content-Length"] == "3"
+
+
+def test_build_raw_request_recomputes_content_length_after_replacement() -> None:
+    components = _components(body="old")
+    new_body = "a much longer replacement body than the original"
+    modified = apply_modifications(
+        components,
+        {"body": new_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 f"Content-Length: {len(new_body.encode('utf-8'))}" in head