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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[Copilot]

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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[Copilot]

There is trailing whitespace at the end of this line. Please remove it to maintain code cleanliness.

Suggested change

	.ok_or_else(|| internal_err!("no leader found in GetMembersResponse"))?;
	.ok_or_else(|| internal_err!("no leader found in GetMembersResponse"))?;

[Copilot]

This empty line appears to contain trailing whitespace and should be removed. The blank line between the error handling and the for loop is not necessary and adds unnecessary whitespace.

Suggested change

	.ok_or_else(|| internal_err!("no leader found in GetMembersResponse"))?;
	.ok_or_else(|| internal_err!("no leader found in GetMembersResponse"))?;

[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.