feat(env): install vendored SGLang via uv path source

Replace pip-resolved sglang==0.5.10 with an editable install from
third_party/sglang/python. The vendored fork carries patches the pip
release does not (admit_direct_append RPC types, _should_allow_local_
prefill_on_decode, maybe_trim_decode_session_cache, backpressure pause
hint) — KVC routing depends on them, so the vendored copy must be the
import target, not just on PYTHONPATH at runtime.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -7,7 +7,7 @@ requires-python = ">=3.12"
 dependencies = [
     "httpx>=0.28.1",
     "mooncake-transfer-engine",
-    "sglang==0.5.10",
+    "sglang",
 ]
 
 [project.scripts]
@@ -22,3 +22,6 @@ where = ["src"]
 
 [tool.uv]
 prerelease = "allow"
+
+[tool.uv.sources]
+sglang = { path = "third_party/sglang/python", editable = true }