Upgrade CI, Docker, and demos to Python 3.14 (#759)

Update all GitHub Actions workflows and Dockerfiles to use Python 3.14
as the default version. Remove the upper bound on requires-python in
model_choice_with_test_harness to allow 3.14+. The CLI's
requires-python stays at >=3.10 for broad compatibility.

demos/samples_python/multi_turn_rag_agent/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.12 AS base
+FROM python:3.14 AS base
 
 FROM base AS builder
 
@@ -9,7 +9,7 @@ RUN pip install --prefix=/runtime --force-reinstall -r requirements.txt
 
 COPY . /src
 
-FROM python:3.12-slim AS output
+FROM python:3.14-slim AS output
 
 COPY --from=builder /runtime /usr/local
 

demos/samples_python/weather_forecast/Dockerfile

@@ -1,7 +1,7 @@
 # Blazing fast Python Docker builds with uv
 
 # The builder image, used to build the virtual environment
-FROM python:3.12 as builder
+FROM python:3.14 as builder
 
 # Install uv
 RUN pip install --no-cache-dir uv
@@ -17,7 +17,7 @@ RUN touch README.md
 RUN uv sync --frozen --no-dev
 
 # The runtime image, used to just run the code provided its virtual environment
-FROM python:3.12-slim as runtime
+FROM python:3.14-slim as runtime
 
 RUN apt-get update && apt-get install -y curl