Update AI endpoint to /ai/joke

[shinyay]

Related to #22

Updates the API endpoint to better align with the intended functionality of generating jokes through Azure OpenAI.

• Renames the endpoint from /prompt to /ai/joke to reflect its purpose more accurately.
• Changes the method name from callAzureOpenAI to generate to clarify the action being performed.
• Modifies the request parameter from prompt to message, aligning with the issue's specification.
• Updates the default message to "Tell me a funny joke in Japanese", making the endpoint's default behavior more specific to generating jokes.

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