Add chat completions support for text_chat detectors

[declark1]

Changes:

• Refactor ToolCall changes merged in Update ToolCall #505
  • Serde wasn't able to correctly identify the ToolCall enum variant using #[serde(untagged) as not all fields are populated for tool call chunks
  • Drop ToolCall enum and separate FunctionToolCall and CustomToolCall variants in favor of single ToolCall struct with optional function and custom fields, so that it can be used for both types.
• Update Message::text(), add Message::has_content() convenience methods
• Add chat completions support for spawning text chat detector tasks
• Add merge_tools_calls() helper to build tool calls from chat completion chunks containing tool call chunks
• Add merge_content() helper to build content from chat completion chunks containing content chunks
• Update no content warning logic and message
  • Dropped ". Output detection was not executed" from warning message as text_chat detections are still executed for this scenario.
• Add text_chat_detectors_with_tools() integration tests for chat completions unary and streaming

Closes #305

[mdevino]

LGTM!