branch-4.1: [fix](group commit) support forward group commit stream load (#63594)

[mymeiyi]

pick #63594

[hello-stephen]

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[Copilot]

Pull request overview

Enables an optional FE-side routing path where group-commit stream load requests received by the master FE can be forwarded to a follower FE (round-robin), improving support for multi-follower deployments (especially in cloud follower mode). Adds a regression test and a new FE config flag to control the behavior.

Changes:

• Add master-side forwarding logic for group-commit stream load PUT requests to follower FEs with a pre-check fallback.
• Introduce enable_forward_group_commit_stream_load_to_follower FE config (master-only, mutable) to toggle forwarding.
• Add a regression test validating group-commit stream loads succeed in a multi-follower setup when forwarding is enabled.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
regression-test/suites/load_p0/stream_load/test_group_commit_stream_load_multi_follower.groovy	Adds a docker-based multi-FE follower-mode regression test for group-commit stream load.
fe/fe-core/src/main/java/org/apache/doris/service/FrontendServiceImpl.java	Implements forwarding of group-commit stream load PUT from master to follower FE (round-robin) with pre-check and fallback.
fe/fe-common/src/main/java/org/apache/doris/common/Config.java	Adds a new config flag to enable/disable forwarding behavior.

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[mymeiyi]

run buildall

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 0.00% (0/51) 🎉
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