[azure-ai-agents] Fix env var name mismatch, typos, and stale link in README

[kkchandi]

This PR fixes several small documentation defects in the azure-ai-agents README that cause copy-paste friction:

• Env var mismatch: the Prerequisites prose says to define PROJECT_ENDPOINT_STRING, but every code snippet and sample reads PROJECT_ENDPOINT (os.environ["PROJECT_ENDPOINT"]), so following the prose leads to a KeyError. Corrected the prose to PROJECT_ENDPOINT.
• Stale docs link: updated the "create a project" link from the legacy azure/ai-studio/ path to the current azure/ai-foundry/ path.
• Grammar: "Role assigned can be done" -> "Role assignment can be done"; "Also notices that if you use asynchronous client ... AsyncFunctionTool be discussed in the later sections" -> "Also note that if you use the asynchronous client ... AsyncFunctionTool will be discussed in later sections".

Docs-only; no code or behavior changes.

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[Copilot]

Pull request overview

Updates azure-ai-agents documentation to reduce copy-paste errors and improve clarity.

Changes:

• Aligns the endpoint environment variable with samples.
• Refreshes the project-creation link.
• Corrects README grammar.

[Copilot]

Pull request overview

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