REST API OpenAPI spec (#612)

* OpenAPI spec in specs/api.  Checked lint with redoc.

specs/api/components/schemas/text-completion/TextCompletionRequest.yaml

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+type: object
+description: |
+  Text completion request - direct LLM completion without RAG.
+required:
+  - system
+  - prompt
+properties:
+  system:
+    type: string
+    description: System prompt that sets behavior and context for the LLM
+    example: You are a helpful assistant that provides concise answers.
+  prompt:
+    type: string
+    description: User prompt or question
+    example: Explain the concept of recursion in programming.
+  streaming:
+    type: boolean
+    description: Enable streaming response delivery
+    default: false
+    example: true

specs/api/components/schemas/text-completion/TextCompletionResponse.yaml

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+type: object
+description: Text completion response
+required:
+  - response
+properties:
+  response:
+    type: string
+    description: Generated text response
+    example: Recursion is a programming technique where a function calls itself...
+  in-token:
+    type: integer
+    description: Number of input tokens consumed
+    example: 45
+  out-token:
+    type: integer
+    description: Number of output tokens generated
+    example: 128
+  model:
+    type: string
+    description: Model used for completion
+    example: gpt-4
+  end-of-stream:
+    type: boolean
+    description: Indicates streaming is complete (streaming mode)
+    default: false
+    example: true