[Sunhy (Shy7777)]

Why are these changes needed?

TextCanvasMemory.clear() replaces the canvas, leaving tools already registered with an agent attached to the old instance. They still report successful writes, but those writes never reach the memory supplied to the next model call. Clear the files and revision history in place so existing update/patch tools and shared canvas references keep working.

Related issue number

Found while testing reuse of a canvas memory after clearing it. The regression tests exercise both existing tools and shared revision history; a separate offline AssistantAgent run also reproduced the lost writes before the fix.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. The clear methods describe the preserved canvas and removed history.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Validation: the two new tests fail before the fix; all 65 related canvas, memory, context, and tool tests pass afterward. Package poe format and poe lint pass, as do mypy and Pyright on the canvas module and its tests. The full extension test task needs optional Playwright dependencies absent from this environment; full-package mypy also reports errors in uninstalled optional integrations. Those broader checks are not claimed as passing.

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