Align readme example and samples for consistency

[anthonychu]

Problem

The first-run story is currently inconsistent across the README quickstart and samples.

Today, the top-level README quickstart presents a minimal OpenAI-based local setup, while the basic-chat sample and deployed sample story are closer to a Microsoft Foundry-based path with provisioned Azure resources. That means new users may see different provider defaults, model settings, and required environment variables depending on whether they start from the README or from samples/basic-chat.

For public preview, the copy/paste path should tell one coherent story.

Proposal

Align the README quickstart and sample first-run guidance around the Foundry-based flow used by basic-chat, or otherwise make the provider choice intentionally consistent across the repo.

Suggested direction:

• Make the top-level README quickstart match the basic-chat sample more closely.
• Use Microsoft Foundry as the primary guided sample path if that is the intended preview experience.
• Keep OpenAI as an alternative path, but present it as an option rather than the primary quickstart if samples default to Foundry.
• Ensure the model/provider environment variables and agents.config.yaml examples line up between the quickstart and samples/basic-chat.

Required updates

• Top-level README quickstart and model-provider setup.
• samples/README.md local development guidance.
• samples/basic-chat/README.md and samples/basic-chat/src/local.settings.template.json if needed.
• Any other sample README/template that describes provider defaults.
• Tests/fixtures only if they encode the sample provider defaults.

Interaction with configuration issues

This should be reconciled with the configuration-direction work in:

• Standardize runtime-owned environment variables under AZURE_FUNCTIONS_AGENTS_* #33, which tracks runtime-owned environment variable naming and overlap with PR Add endpoint-aware agent configuration #15.
• PR Add endpoint-aware agent configuration #15, which proposes agent_configuration as the canonical YAML provider/model/auth surface.

Regardless of which configuration model wins, the public preview quickstart and samples should show the same recommended first-run path.

Acceptance criteria

• A new user following the top-level README quickstart sees the same provider/model story as the primary sample path.
• basic-chat and the README use consistent terminology for Foundry/OpenAI and the required settings.
• OpenAI remains documented if supported, but it is clearly an alternate path if Foundry is the primary guided experience.
• Existing sample templates and READMEs do not contradict the top-level quickstart.
• The issue is resolved in coordination with the final env-var / agent_configuration decision.