Refactor _SharedCache to handle context vs non-context ownership

[shunping]

Currently, _SharedCache registers all live owners as joint owners of every cached subprocess key. This works fine when all owners are within a unified context (e.g. a test class wrapping multiple runs), but it causes unintended resource sharing/leakage when there are concurrent, independent, non-context owners (such as a long-lived runner).

Here is a resource leak scenario:

1. The Prism runner starts and registers a persistent non-context owner owner_1.
2. The pipeline requests an external transform, spinning up a short-lived Java Expansion Service that registers its own owner owner_2 and requests its startup command key.
3. In the old implementation, get() automatically added all currently registered live owners to the cache key's owner set.
   

   beam/sdks/python/apache_beam/utils/subprocess_server.py

   Lines 117 to 118 in 361c2c4

   	for owner in self._live_owners:
   	self._cache[key].owners.add(owner)

   
   As a result, owner_1 (the Prism runner) was registered as a joint owner of the short-lived Expansion Service's cache key.
4. When the pipeline finished using the external transform and called purge(owner_2), the short-lived Expansion Service subprocess was never terminated because owner_1 was still registered as an owner (entry.owners is not empty).
   

   beam/sdks/python/apache_beam/utils/subprocess_server.py

   Lines 104 to 105 in 361c2c4

   	if not entry.owners:
   	to_delete.append(entry.obj)

5. This resulted in orphaned, zombie Expansion Service processes accumulating in the background, binding up localhost ports and exhausting resources.

This PR refactors _SharedCache in subprocess_server.py to explicitly distinguish between context owners (e.g., cache_subprocesses which should hold ownership over everything created under its block) and non-context owners (e.g., individual subprocess server instances like PrismRunner or expansion service subprocesses).

Additionally, get() now takes an optional owner arg. Context owners automatically own all created keys, while independent non-context owners only own the keys they explicitly request.

---

For example, we run YAML example test with pytest apache_beam/yaml/examples/testing/examples_test.py --log-cli-level=INFO -s.

• Without this PR, we see all expansion service servers created in these 46 tests and prism server being destroyed at the end of pytest execution.

  Really destroying service at localhost:49623 with cmd: ('/Users/shunping/.apache_beam/cache/prism/bin/prism', '--job_port', '49623', '--log_level', 'info', '--log_kind', 'json', '--serve_http=false')
  WARNING:apache_beam.utils.subprocess_server:Really destroying service at localhost:49745 with cmd: ['/Users/shunping/.apache_beam/cache/venvs/e08b2501f55abf367196f6578e3dbed1fe2a6d9244fc96ad5d82f3d5c621c1cf/bin/python', '-m', 'apache_beam.runners.portability.expansion_service_main', '--port', '49745', '--fully_qualified_name_glob=*', '--pickle_library=cloudpickle', '--requirements_file=/Users/shunping/.apache_beam/cache/venvs/e08b2501f55abf367196f6578e3dbed1fe2a6d9244fc96ad5d82f3d5c621c1cf-requirements.txt', '--serve_loopback_worker']
  WARNING:apache_beam.utils.subprocess_server:Really destroying service at localhost:49855 with cmd: ['/Users/shunping/.apache_beam/cache/venvs/5052f7801853faa5017d022495ad2a30f1ae0a321fa50de56227e75028f0e612/bin/python', '-m', 'apache_beam.runners.portability.expansion_service_main', '--port', '49855', '--fully_qualified_name_glob=*', '--pickle_library=cloudpickle', '--requirements_file=/Users/shunping/.apache_beam/cache/venvs/5052f7801853faa5017d022495ad2a30f1ae0a321fa50de56227e75028f0e612-requirements.txt', '--serve_loopback_worker']
  WARNING:apache_beam.utils.subprocess_server:Really destroying service at localhost:49888 with cmd: ['/Users/shunping/.apache_beam/cache/venvs/aedc40ba446114c3eb019ef9c411c0ab2204da67a2f4c732530c1c6aba7eef31/bin/python', '-m', 'apache_beam.runners.portability.expansion_service_main', '--port', '49888', '--fully_qualified_name_glob=*', '--pickle_library=cloudpickle', '--requirements_file=/Users/shunping/.apache_beam/cache/venvs/aedc40ba446114c3eb019ef9c411c0ab2204da67a2f4c732530c1c6aba7eef31-requirements.txt', '--serve_loopback_worker']
  

- With this PR, each expansion service server is destroyed as soon as the corresponding test is over. Prism server is cleaned up before pytest ends.