fix: enforce exception assertion in HeartbeatListenerTest (#6594)

[yykaue]

What is this PR for?

Fixes #6594.

This PR replaces the catch-block-only assertion in HeartbeatListenerTest#testSendHeartbeatWithLoginFailure with assertThrows.

The test now:

• Requires Method.invoke to throw an InvocationTargetException.
• Verifies that the underlying cause is a RuntimeException.
• Prevents the test from passing vacuously when no exception is thrown.

Verification

• HeartbeatListenerTest: 8 tests passed.
• Checkstyle passed with no violations.

@Aias00 Could you please help review this PR? Thank you!

[Aias00]

PR #6947 Review — fix: enforce exception assertion in HeartbeatListenerTest (#6594)

Verdict: APPROVE

Scope

shenyu-register-client-beat HeartbeatListenerTest#testSendHeartbeatWithLoginFailure: replace
a catch-block-only assertion with assertThrows.

Verification

• Old code wrapped sendHeartbeatMethod.invoke(...) in try/catch and only asserted inside catch,
  so the test could pass vacuously when no exception was thrown. New code uses
  assertThrows(InvocationTargetException.class, ...) then
  assertInstanceOf(RuntimeException.class, exception.getCause()), enforcing that the failure
  actually occurs and has the right cause.
• Imports java.lang.reflect.InvocationTargetException, assertInstanceOf, assertThrows added
  and used; no production code touched.

Notes

• Pure test-strengthening change; reduces false-green risk. Approving.