fix(build): restore pure-Python uv_build backend to unblock PyPI publish

#31267 switched the build backend to maturin to bundle the Rust OCR
bridge into the litellm wheel. On the release runner, maturin builds
with `--compatibility off` and emits a `cp312-cp312-linux_x86_64`
wheel. PyPI only accepts `manylinux*` Linux wheels, so the upload is
rejected with `400 Bad Request - unsupported platform tag
'linux_x86_64'`, which blocked the 1.91.0.dev2 publish.

Revert `[build-system]` to the pure-Python `uv_build` backend and
restore `[tool.uv.build-backend]` so the release once again produces a
universal `litellm-<ver>-py3-none-any.whl` that installs on every
OS / arch / Python.

The Rust bridge stays optional: `litellm/rust_bridge/loader.py` already
falls back gracefully when `_native` is absent (`try/except
ImportError`), so runtime behavior is unchanged - the native module is
simply not bundled. Rust acceleration can ship as a separate
platform-specific package later.

pyproject.toml

@@ -234,24 +234,8 @@ healthcheck = [
 ]
 
 [build-system]
-requires = ["maturin==1.9.4"]
-build-backend = "maturin"
-
-[tool.maturin]
-manifest-path = "litellm-rust/crates/python-bridge/Cargo.toml"
-module-name = "litellm.rust_bridge._native"
-python-source = "."
-bindings = "pyo3"
-exclude = [
-    "litellm/proxy/enterprise",
-    "litellm/proxy/enterprise/**",
-    "**/__pycache__",
-    "**/__pycache__/**",
-    "**/.pytest_cache",
-    "**/.pytest_cache/**",
-    "**/.ruff_cache",
-    "**/.ruff_cache/**",
-]
+requires = ["uv_build==0.11.8"]
+build-backend = "uv_build"
 
 [tool.uv]
 constraint-dependencies = [
@@ -269,6 +253,18 @@ litellm-enterprise = { workspace = true }
 [tool.uv.workspace]
 members = ["enterprise", "litellm-proxy-extras"]
 
+[tool.uv.build-backend]
+module-root = ""
+source-exclude = [
+    "litellm/proxy/enterprise",
+    "**/__pycache__",
+    "**/__pycache__/**",
+    "**/.pytest_cache",
+    "**/.pytest_cache/**",
+    "**/.ruff_cache",
+    "**/.ruff_cache/**",
+]
+
 [tool.isort]
 profile = "black"