[fix][broker] Fix delayed-delivery bucket merge failures when delayedDeliveryMaxNumBuckets is 1-3

[void-ptr974]

Fixes #26241

Motivation

When delayedDeliveryMaxNumBuckets is set to 1, 2, or 3, the first
bucket-limit overflow contains 2, 3, or 4 immutable buckets. The merge
selector requires more than four buckets and throws IllegalArgumentException,
which fails the merge future and skips that compaction attempt.

The selector also uses an off-by-one timestamp index for the next unloaded
snapshot segment. It can throw when only one unloaded segment remains or select
a merge window using a later segment's timestamp.

Modifications

• Select up to the available number of buckets, bounded by MAX_MERGE_NUM.
• Preserve the existing preference for the largest eligible merge group.
• Use the zero-based timestamp index for the next unloaded snapshot segment.
• Add regression tests for small bucket limits, a single remaining unloaded
  segment, and merge-window timestamp selection.

Verifying this change

• Ran focused BucketDelayedDeliveryTrackerTest merge, segment, and trim
  tests with :pulsar-broker:test -PtestRetryCount=0.
• Ran :pulsar-broker:checkstyleMain :pulsar-broker:checkstyleTest.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

[nodece]

@void-ptr974 Please check conflicts.