Fix MongoDB session lifecycle management in synced adapter

[Copilot]

MongoDB sessions were not being properly cleaned up after transactions, causing session leaks. Sessions were only ended in close(), not after commitTransaction() or abortTransaction().

Changes

Session lifecycle fixes:

• commitTransaction() and abortTransaction() now call session.endSession() and clear the session reference
• getSession() now properly stores newly created sessions in this.session
• All autoCommit/autoAbort code paths updated to end sessions after operations

Type safety:

• Changed session: ClientSession to session?: ClientSession to eliminate unsafe type casts

Usage

The getSession() method is already public and can be used for custom operations within transactions:

const adapter = await MongooseAdapter.newSyncedAdapter('mongodb://...');
await adapter.getTransaction();

// Access session for custom operations
const session = await adapter.getSession();
const CasbinRule = adapter.getCasbinRule();
await CasbinRule.collection.insertOne({ ptype: 'p', v0: 'alice' }, { session });

await adapter.commitTransaction(); // Session properly ended here

Added comprehensive documentation and examples for manual transaction control and session lifecycle patterns.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[feature] better session management for newSyncedAdapter()</issue_title>
<issue_description>Hi！
I am a MongoDB novice and would like to ask two questions. Why doesn’t the newSyncedAdapter API return the current session or call the current session.endSession?
Because I want to perform some operations before submitting the transaction and found that I need to manage the session myself, which doesn’t seem reasonable. Also, I see that people online will call session.endSession after a transaction is completed, but this library only calls session.endSession in the close method. Other APIs do not call session.endSession after the session is committed or discarded. Is there any problem with this? Hope to get your help.</issue_description>

Comments on the Issue (you are @copilot in this section)

@hsluoyz @CheaterScript hi, can you make a PR to return the current session in the function? @hsluoyz @kohtala hi, can you make a PR to fix it?

• Fixes [feature] better session management for newSyncedAdapter() #66

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[hsluoyz]

@nodece @Shivansh-yadav13 please review