[fix][broker] Handle flow_or_qps bundle split requests in ELM

[Denovo1998]

Fixes #xyz

Main Issue: #xyz

PIP: #xyz

Motivation

Manual bundle split requests in the extensible load manager path can fail incorrectly when flow_or_qps_equally_divide is used. The broker was building the split option from the default configured algorithm instead of the algorithm requested by the admin API, which can
trigger a ClassCastException. In addition, when flow_or_qps_equally_divide cannot
produce a valid split boundary, the extensible load manager path turns that no-op case into
a misleading Bundle ... not found under namespace error.

Modifications

• build BundleSplitOption from the requested split algorithm instead of the default broker configuration
• create FlowOrQpsEquallyDivideBundleSplitOption only when the actual requested algorithm is flow_or_qps_equally_divide
• handle the "no valid split boundary" case as a no-op in ExtensibleLoadManagerImpl instead of reporting Bundle ... not found under namespace
• keep the real bundle not found error path only for actual split-bundle lookup failures
• add an admin API regression test to verify FLOW_OR_QPS_EQUALLY_DIVIDE no longer fails when the default split algorithm is different
• add an extensible load manager regression test to verify the no-boundary case does not fail and leaves the bundle layout unchanged

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: apache#25442