build: raise requires-python cap to <3.15 so Python 3.14 installs current releases (#33438)

* build: drop requires-python upper cap so Python 3.14 resolves to current releases

The <3.14 cap made pip on Python 3.14 fall back to litellm 1.83.7, a
pre-April release whose old auth flow fails with 400s. The cap was added
in d9a460277a because deps lacked 3.14 wheels and uv could not resolve
the 3.14 split; both are fixed now via the existing python_version
markers plus a ddtrace version split (2.x has no cp314 wheels, 3.16+
does). Verified on 3.14.5: uv sync --all-extras installs, litellm and
proxy_server import (rust bridge falls back to pure python), real
provider calls succeed sync/async/streaming, and the core-utils test
suite passes.

* build: cap requires-python at <3.15 and keep ddtrace on one major per python band

Reviewer preference to bound the supported window at the newest tested
minor rather than leaving it open-ended, and Greptile flagged the
ddtrace 3.14+ range spanning two majors; every ddtrace 4.x ships cp314
wheels so the band is now >=4.0,<5.0, matching the single-major
convention of the 2.x band.

pyproject.toml

@@ -3,7 +3,7 @@ name = "litellm"
 version = "1.94.0"
 description = "Library to easily interface with LLM API providers"
 readme = "README.md"
-requires-python = ">=3.10, <3.14"
+requires-python = ">=3.10, <3.15"
 license = "MIT"
 license-files = ["LICENSE"]
 authors = [
@@ -129,7 +129,8 @@ proxy-runtime = [
     "opentelemetry-sdk==1.28.0",
     "opentelemetry-exporter-otlp==1.28.0",
     "opentelemetry-instrumentation-fastapi==0.49b0",
-    "ddtrace>=2.19.0,<3.0",
+    "ddtrace>=2.19.0,<3.0; python_version < '3.14'",
+    "ddtrace>=4.0.0,<5.0; python_version >= '3.14'",
     "sentry-sdk>=2.21.0,<3.0",
     "mangum>=0.17.0,<1.0",
     "azure-ai-contentsafety>=1.0.0,<2.0",