public/service: expose collapsed count helpers for plugin developers

[twmb]

Summary

• Adds Message.WithCollapsedCount(count int) method to indicate a message is the result of collapsing multiple messages into one
• Adds MessageBatch.CollapsedCount() method to get the total collapsed count across a batch
• These wrap the internal batch.CtxWithCollapsedCount / batch.CtxCollapsedCount functions, enabling plugin developers to properly set collapsed counts for accurate output_sent / output_batch_sent metrics when implementing batch-collapsing processors (like archive)
• Count is accumulative: calling WithCollapsedCount(3) then WithCollapsedCount(2) yields 4 (base + count - 1 to avoid double-counting)

Test plan

• Unit test for WithCollapsedCount and CollapsedCount covering default, set, accumulation, and multi-message batch
• All existing public/service tests pass

Closes redpanda-data/connect#1877

🤖 Generated with Claude Code

[claude]

Commits
LGTM

Review
Clean public API wrappers around existing internal batch.CtxWithCollapsedCount / batch.CtxCollapsedCount. The implementation mirrors the internal MessageCollapsedCount pattern and tests cover default, accumulation, and multi-message batch scenarios.

LGTM

[mmatczuk]

API change @Jeffail PTAL

[claude]

Commits
LGTM

Review
Adds WithCollapsedCount and CollapsedCount as thin public wrappers over the internal batch.CtxWithCollapsedCount/batch.CtxCollapsedCount functions. The delegation is correct, accumulation semantics match the internal implementation, and the test covers default, set, accumulation, and multi-message batch scenarios.

LGTM