feat: Add agent binding interface

[hallvictoria]

Adds lightweight, provider-neutral Agent APIs to azure-functions:

• Adds FunctionApp.markdown_agent() for decorator-based Agent injection.
• Adds and exports AiApp and DurableAiApp.
• Loads Agent extensions lazily, keeping provider frameworks and Durable Functions out of core imports.
• Reports actionable installation guidance when a provider or Durable extra is missing.
• Delegates Agent construction, markdown resolution, execution, and lifecycle management to extension packages.
• Adds API documentation and focused delegation/error-handling tests.

These APIs add no host binding metadata and allow future Agent providers through the shared extension contract.

[Copilot]

🟡 Changes recommended

There is a confirmed provider-selection logic bug in AiApp.markdown_agent, and the new public markdown_agent API lacks consistent type annotations.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds provider-neutral “agent binding” entry points to the Azure Functions Python programming model, enabling extension packages to supply agent behavior (and optional Durable support) without the core SDK depending on a specific provider implementation.

Changes:

• Introduces FunctionApp.markdown_agent(...) plus AiApp/DurableAiApp convenience wrappers that delegate to a provider extension module.
• Exports AiApp and DurableAiApp via azure.functions.decorators and azure.functions.
• Documents the new APIs in README.md and docs/ProgModelSpec.pyi, and adds unit tests validating delegation/error behavior.

File summaries

File	Description
tests/decorators/test_agents.py	Adds unit tests for agent app delegation, configuration, exports, and import error messaging.
README.md	Documents provider-neutral agent APIs and durable extra installation guidance.
docs/ProgModelSpec.pyi	Updates public API spec with markdown_agent, AiApp, and DurableAiApp signatures.
azure/functions/decorators/function_app.py	Implements provider-loading, markdown_agent, and new AiApp/DurableAiApp classes.
azure/functions/decorators/init.py	Re-exports AiApp/DurableAiApp from decorators package.
azure/functions/init.py	Re-exports AiApp/DurableAiApp at top-level azure.functions.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 2
• Review effort level: Lite

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