ci: lock the reflex 0.8.23 playground job and run examples only when they can change (#7569)

* ci: lock the reflex 0.8.23 playground job and run it only when it can change

The playground-0.8.23 job installed reflex 0.8.23 with unpinned transitive
dependencies, so a new SQLAlchemy release (2.1, which drops greenlet from its
core dependencies) broke it on every PR. Resolve its environment from a
uv.lock under .github/legacy/reflex-0.8.23 instead, with greenlet declared
for SQLAlchemy 2.1.

Changes to reflex itself cannot affect a published release, so move the job
into its own advisory workflow, examples-legacy, triggered only by the
playground, reflex-bench, the locked environment or the workflow itself.

Claude-Session: https://claude.ai/code/session_01UvPWTzLh6vkVatxxRMej6E

* ci: run the examples workflow only for example and reflex-bench changes

Drop reflex/**, packages/**, pyproject.toml, uv.lock and the build env action
from the examples workflow's path filters, so pull requests that do not touch
the playground, reflex-bench or the locked 0.8.23 environment skip it. Fold the
playground-0.8.23 job back into examples.yml instead of a separate workflow.

Claude-Session: https://claude.ai/code/session_01UvPWTzLh6vkVatxxRMej6E

Author: FarhanAliRaza

.github/legacy/reflex-0.8.23/pyproject.toml

@@ -0,0 +1,12 @@
+[project]
+name = "reflex-legacy-0-8-23"
+version = "0.0.0"
+description = "Locked reflex 0.8.23 environment the playground-0.8.23 CI job compiles examples/playground with."
+requires-python = ">=3.12,<3.13"
+# plotly: rx.plotly imports it when it renders. greenlet: reflex 0.8.23 imports
+# sqlalchemy.ext.asyncio at startup, and SQLAlchemy 2.1 only installs greenlet
+# with its asyncio extra.
+dependencies = ["reflex==0.8.23", "plotly>=6.0,<7.0", "greenlet"]
+
+[tool.uv]
+package = false