Add context provider middleware

[qmuntal]

Port microsoft/agent-framework#4097.

[Copilot]

Pull request overview

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This PR introduces a reusable context-provider middleware and wires agent context providers through the middleware chain, while keeping local in-memory history as a fallback when no explicit context providers are configured.

Changes:

• Added middleware/contextprovider package to run ContextProvider hooks before/after agent runs.
• Updated agent.New middleware wiring to use the new context-provider middleware and conditionally apply local history defaults.
• Added unit tests covering message/tool enrichment and AfterRun behavior.

Show a summary per file

File	Description
middleware/contextprovider/contextprovider.go	New middleware that runs providers before run and persists after run
middleware/contextprovider/contextprovider_test.go	New tests for message/tool enrichment and AfterRun semantics
agent/agent.go	Switches agent wiring to the new middleware and adds default local-history fallback

Copilot's findings

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• Files reviewed: 3/3 changed files
• Comments generated: 4