Added CHANGELOG entry describing new thinking-related functionality

changelog/3175.added.md

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+- Added additional functionality related to "thinking", for Google and Anthropic
+  LLMs.
+
+  1. New typed parameters for Google and Anthropic LLMs that control the
+     models' thinking behavior (like how much thinking to do, and whether to
+     output thoughts or thought summaries):
+     - `AnthropicLLMService.ThinkingConfig`
+     - `GoogleLLMService.ThinkingConfig`
+  2. New frames for representing thoughts output by LLMs:
+     - `LLMThoughtStartFrame`
+     - `LLMThoughtTextFrame`
+     - `LLMThoughtEndFrame`
+  3. A generic mechanism for associating extra LLM-specific data with a function
+     call in context, used specifically to support Google's function-call-related
+     "thought signatures", which are necessary to ensure thinking continuity
+     between function calls in a chain (where the model thinks, makes a function
+     call, thinks some more, etc.). See:
+     - `FunctionCallInProgressFrame.llm_specific_extra`
+     - `LLMAssistantAggregator` handling of the above field
+     - `GeminiLLMAdapter` handling of`"tool_call_extra"` context messages
+  4. A generic mechanism for recording LLM thoughts to context, used
+     specifically to support Anthropic, whose thought signatures are expected to
+     appear alongside the text of the thoughts within assistant context
+     messages. See:
+     - `LLMThoughtEndFrame.signature`
+     - `LLMAssistantAggregator` handling of the above field
+     - `AnthropicLLMAdapter` handling of `"thought"` context messages
+  5. A generic mechanism for recording standalone thought signatures to context,
+     used specifically to support Gemini 3 Pro, which may return
+     non-function-call-related thought signatures at the end of
+     non-function-call assistant responses. See:
+     - `LLMThoughtSignatureFrame`
+     - `LLMAssistantAggregator` handling of the above frame
+     - `GeminiLLMAdapter` handling of `"thought_signature"` context messages
+  6. An expansion of `TranscriptProcessor` to process LLM thoughts in addition
+     to user and assistant utterances. See:
+     - `TranscriptProcessor.thought()`
+     - `ThoughtTranscriptionMessage`, which may now also be emitted with the
+       `"on_transcript_update"` event