Warn when a copy of pipeline.blocks is modified #15067

ModularPipeline.blocks returns a copy, so modifying it (e.g. pipe.blocks.sub_blocks.insert(...)) does not change the pipeline. This is intentional and documented (see the note under init_pipeline), but some users may assume they can modify the pipeline this way (see #12386). It would be better to warn and point to the correct approach: modify the blocks, then create a new pipeline with blocks.init_pipeline().

Posted by Claude Code on behalf of @yiyixuxu, with their approval.