concurrency: add timeout for LeaseGrant in NewSession

[qts0312]

This PR follows concurrency: add timeout for LeaseGrant in NewSession #20022 as the original PR has no updates and been closed now. The credit should belong to @Antraxmin.

What this PR does

This PR adds a timeout to the LeaseGrant RPC call in the NewSession function to prevent infinite blocking.

• Fixes Missing deadline for LeaseGrant when creating session #19778

Why this change is needed

Currently, LeaseGrant in NewSession is called without a deadline, which can cause the function to block indefinitely if the RPC call hangs. This is inconsistent with the Close() method, which properly uses context.WithTimeout for the LeaseRevoke call.

Changes made

• Modified NewSession function in client/v3/concurrency/session.go to use context.WithTimeout for LeaseGrant call
• The timeout is set to defaultLeaseTimeout seconds
• Added integration tests to verify timeout behavior

Testing

• All existing integration tests pass
• Added new tests to verify timeout behavior works correctly
• Verified normal session creation still works as expected

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @qts0312. Thanks for your PR.

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[serathius]

Please sign DCO

[serathius]

There is already tests/integration/clientv3/concurrency/session_test.go that has only 100 lines, don't think we need a separate file for timeout.

[qts0312]

I have simplified the tests and moved them to session_test.go