feat: Use per-context replay status

[ayushiahjolia]

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Issue Link, if available

Fixes #383

Adds per-context replay tracking to the Java SDK. This is a prerequisite for the plugin hooks feature (#364) and OTel integration (#333).

Problem

Log suppression during replay used the global ExecutionManager.isReplaying() flag. When multiple branches run concurrently (parallel/map), the global flag flips to EXECUTION as soon as any branch encounters new work - causing logs in other branches that are still replaying to leak through.

The old per-context field (BaseContextImpl.isReplaying) was a one-shot transition guard used only by BaseDurableOperation.execute(). It was never used for log suppression and answered a different question ("has this context ever seen a non-cached op?") rather than ("is the next operation already checkpointed?").

Solution

Added proactive per-context replay tracking in DurableContextImpl:

• replayMode field - initialized by checking if the next operation (via peekNextOperationId()) exists in checkpoint storage
• updateReplayStatus() - called after nextOperationId() in each operation method; peeks at the following operation and transitions out of replay if it doesn't exist
• isReplayingContext() - exposes the per-context replay status to the logger and future plugin hooks

This matches the Python SDK's _track_replay() pattern: after consuming the current operation's ID, peek at the next one - if it's not in storage, we're past the replay boundary.

Checklist

• I have filled out every section of the PR template
• I have thoroughly tested this change

Testing

Unit Tests

Have unit tests been written for these changes? Yes

Integration Tests

Have integration tests been written for these changes? N/A

Examples

Has a new example been added for the change? (if applicable) N/A