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

[Yvan Wang (BootstrapperSBL)]

Fixes #36931

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Five # type: ignore comments in AIMessage.content_blocks, AIMessageChunk.init_tool_calls, and AIMessageChunk.init_server_tool_calls were silencing mypy rather than addressing what it was complaining about. Swap each one for a cast("dict[str, Any]", ...) placed at the site where narrowing is actually needed.

For the typeddict-item sites the source ToolCall TypedDict in messages.tool does not declare index or extras, even though they exist at runtime — casting the source tool_call to dict[str, Any] lets mypy read those keys without silencing the diagnostic. For the three index sites, self.content[idx] is typed str | dict[str, Any] and mypy cannot narrow it from the outer isinstance(block, dict) check on a separate loop variable — casting the target to dict[str, Any] at the assignment point expresses the intent directly.

Runtime behavior is identical. The two load-bearing ignores called out in the issue (the type field override on AIMessageChunk and the __add__ overload) are left in place — they encode real architectural choices, not suppressed bugs.

Before:

tool_call_block["index"] = tool_call["index"]  # type: ignore[typeddict-item]

After:

tool_call_block["index"] = cast("dict[str, Any]", tool_call)["index"]

mypy langchain_core and mypy tests both still come back clean, and the full libs/core unit test suite passes locally.

Disclosure: this contribution was prepared with assistance from an AI coding agent; a human reviewed the change before opening the PR.

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