[4.x] Fix Lambda permission SourceArn for the default event bus

[GrahamCampbell]

When an eventBridge event is configured with an explicit eventBus: default, the generated AWS::Lambda::Permission included a spurious default/ segment in its SourceArn condition, producing rule/default/<rule-name>. AWS assigns rules on the default bus an ARN with no bus segment (rule/<rule-name>), so the ArnLike condition never matched and EventBridge was silently denied invoking the function on every invocation, with no error at deploy time. Events with eventBus omitted and named or custom buses were unaffected.

The SourceArn now omits the bus segment for the default bus, mirroring the existing special case for EventBusName on the rule resource. The default-bus tests now assert the full SourceArn value instead of a single path segment, and coverage is added for a custom-named rule on the default bus. Fixes #426 and supersedes #427; thanks to @james-at-tem for the diagnosis and the original patch, credited as co-author.