Amazon S3 event notifications delivered to SNS, SQS, and Lambda now
include an `awsGeneratedTags` map on the `bucket` object when system
tags are enabled on the source bucket. The Java SDK model did not
round-trip this field, so consumers had to fall back to raw JSON
parsing to read the tags.

This change adds `Map<String, String> awsGeneratedTags` to `S3Bucket`
and threads it through the reader and writer:

  * `S3Bucket`: adds `awsGeneratedTags` field with a getter, a new
    four-argument constructor, and updates `equals`/`hashCode`/
    `toString`. The pre-existing three-argument constructor is
    preserved so existing callers are source- and behavior-compatible.
    Tags are defensively copied on construction and returned as an
    unmodifiable view.
  * `DefaultS3EventNotificationReader`: parses `awsGeneratedTags` as
    `Map<String, String>`. Absent field yields `null`. Null value
    entries are preserved. Non-string values throw
    `IllegalArgumentException` with a helpful message including the
    offending key and value.
  * `DefaultS3EventNotificationWriter`: emits `awsGeneratedTags` only
    when the map is non-null and non-empty, matching S3's server-side
    `@JsonInclude(NON_EMPTY)` behavior so round-tripped output is
    indistinguishable from what S3 emits.

Also bumps `eventVersion` fixtures in the module test files from
`2.4` to `2.5` to reflect the schema version emitted alongside this
launch.

The change is fully additive:
  * Older SDKs remain compatible with new payloads (unknown fields
    are ignored during parse).
  * Newer SDKs remain compatible with old payloads
    (`getAwsGeneratedTags()` returns `null` when the field is absent).