fix(core): remove 5 unnecessary type: ignore comments in ai.py

### Submission checklist - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [x] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs https://github.com/langchain-ai/langchain/issues/17048 ### Reproduction Steps / Example Code (Python) ```python # From libs/core/, run: uv run --group lint mypy langchain_core/messages/ai.py # # Then manually remove any of the 5 type: ignore comments below and re-run mypy # to see the errors they suppress: # # In AIMessage.content_blocks (2 ignores): # tool_call_block["index"] = tool_call["index"] # type: ignore[typeddict-item] # tool_call_block["extras"] = tool_call["extras"] # type: ignore[typeddict-item] # # In AIMessageChunk.init_tool_calls (1 ignore): # self.content[idx]["extras"] = block["extras"] # type: ignore[index] # # In AIMessageChunk.init_server_tool_calls (2 ignores): # self.content[idx]["type"] = "server_tool_call" # type: ignore[index] # self.content[idx]["args"] = args # type: ignore[index] #…