fix: set up constant environment variable for prompt

[Priyanka-Microsoft]

Purpose

This pull request introduces a new fallback prompt for SQL agent operations, enhancing the robustness of the system when generating T-SQL queries. The changes span infrastructure configuration, environment variables, and backend logic to integrate the new fallback prompt seamlessly.

Infrastructure Updates:

• Added a new parameter sqlAgentFallbackSystemPrompt to infra/deploy_app_service.bicep with a description for the SQL agent fallback system prompt.
• Updated the Website resource in infra/deploy_app_service.bicep to include the new environment variable SQL_AGENT_FALLBACK_PROMPT.
• Defined the sqlagentFallbackSystemPrompt variable in infra/main.bicep to specify the fallback prompt content.
• Passed the sqlAgentFallbackSystemPrompt variable to the appserviceModule in infra/main.bicep.

Environment Configuration:

• Added SQL_AGENT_FALLBACK_PROMPT to src/App/.env.sample with detailed rules and schema for generating T-SQL queries.

Backend Logic:

• Modified agent_factory.py to use SQL_AGENT_FALLBACK_PROMPT as a fallback when SQL_SYSTEM_PROMPT is unavailable.
• Updated common/config.py to include SQL_AGENT_FALLBACK_PROMPT as a new configuration property sourced from environment variables.

Does this introduce a breaking change?

• Yes
• No

Golden Path Validation

• I have tested the primary workflows (the "golden path") to ensure they function correctly without errors.

Deployment Validation

• I have validated the deployment process successfully and all services are running as expected with this change.

What to Check

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