[runtime][python] Refresh imports for new dependency generations

[joeyutong]

Linked issue: #941

Purpose of change

Flink may rematerialize a job's Python dependencies under a new python-dist-* directory after task failover, while Pemja's process-level main interpreter and its import state remain alive in the TaskManager JVM. Existing user modules can therefore keep __file__, __path__, or __spec__ entries that point to the removed dependency directory.

This change activates each job's current dependency generation before loading Python actions or resources. When the generation changes, it:

• clears the Flink Agents Python function cache;
• evicts modules attributed to the previous generation from sys.modules;
• removes stale sys.path and importer-cache entries;
• prepends the current generation's PYTHONPATH entries; and
• preserves modules and paths owned by other active jobs.

The generation is recorded only after the refresh succeeds, so a failed refresh is retried on the next initialization.

Tests

• uv run --project python pytest -q python/flink_agents/runtime/tests/test_python_dependency.py (3 passed)
• mvn -B --no-transfer-progress -pl runtime -am -Dtest=PythonDependencyGenerationManagerTest -Dsurefire.failIfNoSpecifiedTests=false test (1 passed)
• Maven Spotless formatting/checks passed as part of the Java verification

API

No public API changes.

Documentation

• doc-needed
• doc-not-needed
• doc-included