feat(aws): Enable Buffering and Flushing Writes to Checkpoint Savers

[Farouk Boukil (f4roukb)]

Issue: #806

Description: Enable buffering the write ops of any checkpoint saver and flushing them manually where needed (e.g., inside the graph execution), and automatically at the end of graph execution.

Changes:

• New BufferedCheckpointSaver class (decorator pattern with both sync and async API). It buffers writes and flushes them:
  a) After the context is exit when a context manager is used: aflush_on_exit (async) or flush_on_exit (sync).
  b) When the aflush (async) or flush (sync) method is called (potentially inside the graph).
• Unit tests on top of Langgraph's MemorySaver, including tests for edge cases.
• Component tests for buffering on top of AgentCoreMemorySaver, AgentCoreValkeySaver, and DynamoDBSaver.
  I essentially picked the existing component tests back up and adapted them by wrapping the instances in the buffering decorator and adapting the assertions.
  I also added more fundamental component tests in each of these test suites to ensure behavior is tested on different kinds of graphs, with streaming, with parallelism, with error mid-execution, and with flush mid-execution.
• There is some redundancy across component tests. I tried to minimize it by adding conftest.py and utils.py files. I haven't modified the existing component tests. A clean-up could be scheduled if this PR is merged.
• No breaking changes.

[Farouk Boukil (f4roukb)]

Michael Chin (@michaelnchin), since you addressed the corresponding issue, could you please have a look? Happy to address your comments. Thank you.

[Michael Chin (michaelnchin)]

Thank you for the submission Farouk Boukil (@f4roukb) !

Added some feedback below. Please also add additional unit/integration tests to validate these edge cases where needed.

[Michael Chin (michaelnchin)]

Even if there's no danger to flushing the existing buffer on exceptions, we should still log them for visibility given that the resumed checkpoint may not behave as expected due to the ignored failure.

[Farouk Boukil (f4roukb)]

I added some log messages there. Let me know what you think.

[Farouk Boukil (f4roukb)]

I missed quite a few things working on this one, huh 😅 Thanks a lot for the spot-on observations. I addressed your comments and will be working on additional unit/integration tests. Should be re-reviewable by EOW.

[Farouk Boukil (f4roukb)]

I think it's a good milestone to get some more feedback please. I updated the PR description with more details. I can add component tests for the AsyncBedrockSessionSaver afterwards. 😄

Please note that I don't have access to the necessary infrastructure to actually run and debug the component tests. I'd really appreciate it if you could help push this. Thank you.