fix(core): deduplicate tool_calls in content_blocks with OpenAI Responses API streaming

[Anmol Mishra (anmolxlight)]

Closes #36985

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Problem

When using the OpenAI Responses API, streaming produces partial tool_call blocks in content (fc_ prefix, empty args) while tool_calls holds the finalized blocks (call_ prefix, full args). The previous ID-based dedup logic in AIMessage.content_blocks did not recognize these as the same calls, resulting in duplicate entries.

Root cause

The old logic at AIMessage.content_blocks compared tool_call IDs between content and self.tool_calls:

content_tool_call_ids = {block.get("id") for block in self.content ...}
for tool_call in self.tool_calls:
    if id_ not in content_tool_call_ids:
        blocks.append(tool_call_block)  # always added, IDs differ

Since fc_12345 != call_abc123, both the partial and finalized blocks ended up in content_blocks.

Fix

Match tool_call blocks by name and position instead of ID. When a tool_call block from content matches a tool_call from self.tool_calls at the same position (same name), replace the partial with the finalized version. Any remaining tool_calls not represented in content are appended.

This preserves the existing behavior for:

• Tool_calls already present in content with matching IDs (no duplicate added)
• Tool_calls only in self.tool_calls (appended as before)
• Multiple tool_calls with the same name (matched by position)

Changes

• libs/core/langchain_core/messages/ai.py -- rewritten dedup logic in AIMessage.content_blocks
• libs/core/tests/unit_tests/messages/test_ai.py -- two new regression tests:
  • test_content_blocks_no_duplicate_tool_calls_openai_responses_streaming -- verifies the fc_/call_ dedup
  • test_content_blocks_no_duplicate_tool_calls_mixed_content -- verifies matching-ID dedup still works

All 200 existing message unit tests continue to pass.

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This contribution was developed with the assistance of an AI coding agent.

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