Python: Prevent compaction from emitting empty projections

[ronronner02]

Motivation & Context

Fixes #7213.

Compaction must not return an empty model-visible projection when the newest message group alone exceeds the target budget. An empty projection causes providers to reject an otherwise healthy agent run.

Description & Review Guide

• What are the major changes? Add a shared floor calculation that retains the latest included non-system group, or the last system group when no non-system group exists. Apply it to TruncationStrategy and both fallback paths in TokenBudgetComposedStrategy.
• What is the impact of these changes? Oversized final groups remain visible instead of being dropped, so the projection can remain over budget but stays valid and actionable. Existing cases that can reach their target budget continue to do so.
• What is the intentional trade-off? When the minimum-retained group alone exceeds the budget, the strict fallback may remove system anchors rather than remove that final group. The budget remains over target, but the projection is non-empty and preserves the newest model-visible context; this is preferable to sending an invalid empty request.
• What do you want reviewers to focus on? The floor behavior when system-only context, a single oversized group, or an already-compacted message list reaches either fallback path.

The patch was developed with AI assistance and manually reviewed against the existing compaction annotations and test conventions.

Related Issue

Fixes #7213

Contribution Checklist

• The changed module builds clean without any errors or warnings; package-wide Pyright is currently blocked by pre-existing optional dependency resolution errors in unrelated modules (_workflows/_viz.py and observability.py).
• Focused unit tests and the complete packages/core/tests suite pass; regression tests were added.
• Ruff lint/format, MyPy for the changed test and source files, and Pyright for the changed source module pass.
• The PR follows the Contribution Guidelines.
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change.

[Copilot]

Pull request overview

This PR fixes a Python compaction edge case where truncation and token-budget fallback paths could exclude every model-visible message (or every non-system message), producing an empty projection that providers reject. It introduces a shared “minimum retained group” floor so the newest included non-system group (or the last system group if no non-system group exists) is always kept.

Changes:

• Add _minimum_retained_group_ids(...) and apply it to TruncationStrategy protection logic.
• Apply the same floor to TokenBudgetComposedStrategy fallback and strict-fallback exclusion loops.
• Add/adjust regression tests in test_compaction.py to cover oversized-latest-group and system-only scenarios.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
python/packages/core/agent_framework/_compaction.py	Adds and applies a minimum-retained-group floor to prevent empty projections during truncation and token-budget fallback paths.
python/packages/core/tests/core/test_compaction.py	Updates token-budget thresholds and adds regression tests for “keep latest group” / system-only behaviors.

[ronronner02]

@microsoft-github-policy-service agree

[github-actions]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_compaction.py	710	58	91%	117–118, 126, 193–194, 212–213, 230, 264, 278, 285, 300–301, 355, 362, 378, 428, 452, 483, 532, 538, 540, 559, 603–608, 620, 704, 706, 721, 766, 828, 954, 960–964, 967–969, 977, 1052, 1054, 1070, 1077, 1082, 1097, 1105, 1204, 1227, 1239, 1335, 1362, 1446
TOTAL	44842	5102	88%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
9140	33 💤	0 ❌	0 🔥	2m 25s ⏱️