Restore include_examples parameter to DagBag for backwards compatibility

[bramhanandlingala]

Summary

include_examples was silently removed from DagBag.__init__ and DagBag.collect_dags() in a minor version bump (3.1.8 -> 3.3.0), breaking any code that constructs DagBag directly with this argument — a common pattern in custom scripts and DAG validation tests — with a TypeError: __init__() got an unexpected keyword argument 'include_examples'. This wasn't mentioned in the 3.3.0 release notes and violates semver expectations for a minor release.

Root Cause

Example DAGs are now loaded via the DAG bundle system (DagBundlesManager) instead of DagBag scanning airflow.example_dags directly. That architectural change is reasonable on its own, but the include_examples parameter was removed outright instead of being deprecated, so any caller passing it — whether True or False — hits a hard crash instead of a warning.

Fix

Restores include_examples to DagBag.__init__ and DagBag.collect_dags(), along with BundleDagBag's guard against include_examples=True. When the parameter is omitted (the common case), behavior is unchanged and no warning is emitted. When passed explicitly, it still works as it did pre-3.3.0 but now emits a DeprecationWarning pointing callers to the new DAG bundle system instead of crashing. Added TestDagBagIncludeExamples in test_dagbag.py covering the omitted, explicit-True, explicit-False, and BundleDagBag cases.

Fixes #70283