fix: replace panic unwrap with error check in cluster.rs (#504)

[SaathwikDasari]

What is changed and how it works?

Replaced a panic-inducing .unwrap() on resp.leader with ok_or_else in src/pd/cluster.rs.

If resp.leader is None, the client will now return a proper error instead of crashing the entire application.

Issue reference

Closes #504

Check List

• Code is formatted (ran cargo fmt? If not, run it locally now!)
• This PR is signed-off

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Pull request overview

This PR addresses a critical panic issue in the TiKV client library where calling .unwrap() on resp.leader caused application crashes when the leader was None. The fix replaces the panic-inducing unwrap with proper error handling using ok_or_else.

Key changes:

• Replaced .unwrap() with .ok_or_else() to return a proper error instead of panicking
• Added descriptive error message "no leader found in GetMembersResponse"

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[pingyu]

The resp.leader must not be None. Otherwise, Connection::try_connect (and Connection::connect) will return error (see #452).

But I think it's still nice to have this change to make it more robust.

@SaathwikDasari Please fix the error of "CI / check". Rest LGTM.

[pingyu]

cc @iosmanthus

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: iosmanthus, pingyu

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• OWNERS [iosmanthus,pingyu]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-04 01:34:12.379354884 +0000 UTC m=+493208.197663326: ☑️ agreed by pingyu.
• 2026-01-05 09:14:31.803810942 +0000 UTC m=+607227.622119384: ☑️ agreed by iosmanthus.

[pingyu]

@SaathwikDasari Thanks for your contribution !