fix(events): serialize EventBusPolicy statements

[MukundaKatta]

Issue

Closes #24031.

Reason for this change

EventBusPolicyProps.statement is typed as iam.PolicyStatement, but EventBusPolicy passed the PolicyStatement instance directly into the underlying CfnEventBusPolicy. When that statement includes principals, CloudFormation synthesis can walk the grantPrincipal object graph and fail with a circular reference.

EventBus.addToResourcePolicy() already avoids this by passing statement.toJSON() into EventBusPolicy. Direct EventBusPolicy usage should do the same thing.

Description of changes

• Serializes EventBusPolicyProps.statement with toJSON() before passing it to CfnEventBusPolicy.
• Adds a regression test covering direct EventBusPolicy usage with an AccountPrincipal.

Description of how you validated changes

• git diff --check passes.
• Attempted to run jest aws-events/test/event-bus.test.ts --runInBand using the existing local AWS CDK toolchain. The test runner could not start in the fresh worktree because generated core/dist artifacts were missing (../dist/core/cfn-utils-provider.generated).

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES
• My change is relevant to the issue above
• I added a regression test
• I tried to run relevant tests and documented the local environment blocker

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

✅ A exemption request has been requested. Please wait for a maintainer's review.

[MukundaKatta]

Exemption Request: this fix is limited to serializing EventBusPolicyProps.statement before synthesis. The regression test covers the failing direct EventBusPolicy path with an AccountPrincipal and asserts the synthesized statement JSON. An integration test would only snapshot the same synthesized policy shape, so the unit regression is the targeted coverage for this bug.