Add true time based flush behaviour to Coalesce transform

[tharinduamila-insta]

Closes : #1673

This PR introduces a background timer that will force the Transfrom chain to run if a time based flush definition is present when the time since last flush is greater than the time defined.

Last flush is considered to be the time

1. When messages were truely flushed due meeting some condition.
2. When the chain runs and the messages to flush is empty.

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks
⏩ 2 skipped benchmarks¹

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_{Comparing tharinduamila-insta:1673-implement-true-flush_when_millis_since_last_flush (0443b3e) with main (e66d846)}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Copilot]

Pull request overview

This PR adds true time-based flushing to the Coalesce transform by introducing a per-connection background timer that can wake the chain to flush buffered messages, rather than only flushing opportunistically when new messages arrive.

Changes:

• Implement a per-connection Tokio task in Coalesce that periodically signals force_run_chain and triggers a flush when the buffer is non-empty.
• Update config validation/error messaging around flush settings (including requiring millis > 0 to enable the timer).
• Refresh tests and documentation to reflect the new timer-driven behavior and semantics.

Reviewed changes

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

File	Description
shotover/src/transforms/coalesce.rs	Adds timer-driven flush behavior, new validation logic, and expanded tests for timer/count interactions.
shotover/src/config/topology.rs	Updates topology validation test expectations to match new Coalesce validation messaging.
docs/src/transforms.md	Documents the new per-connection timer behavior and clarifies configuration semantics.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

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[justinweng-instaclustr]

The additional comments here look good and clear. Have approved the PR

[yallen-ic]

LGTM