Python: Implement annotation-based context compaction

[eavanvalkenburg]

Summary

• implement annotation-based context compaction primitives and built-in strategies in packages/core/agent_framework/_compaction.py
• wire compaction through BaseChatClient and simplify FunctionInvocationLayer so compaction happens in the chat client layer
• add and update core compaction/function-loop tests, plus compaction samples under samples/02-agents/compaction

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_agents.py	349	41	88%	436, 440, 492, 867, 903, 919, 1002–1005, 1068, 1190, 1206, 1208, 1221, 1227, 1263, 1265, 1274–1279, 1284, 1286, 1292–1293, 1300, 1302–1303, 1311–1312, 1315–1317, 1325–1326, 1328, 1333, 1335
_clients.py	113	10	91%	309, 355, 464–467, 554, 556, 561, 565
_compaction.py	462	56	87%	102, 148–160, 167–169, 175–176, 191–194, 201–203, 249, 273, 283, 345, 348, 350, 369, 411–416, 428, 500, 502, 517, 621, 642, 703, 705, 721, 728, 733, 748, 756, 846, 849, 867, 883
TOTAL	23040	2861	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
4607	25 💤	0 ❌	0 🔥	1m 21s ⏱️