Fix duplicate AI messages for parallel tool calls (#1463)

[VishnuVV27]

PR Checklist

• Read the Contributing documentation.
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• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages: agents: fix duplicate AI messages for parallel tool calls
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• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves: Fixes How Should Parallel Tool Calls Be Grouped in OpenAIFunctionsAgent? #1463
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Description

This PR resolves issue #1463 where the OpenAIFunctionsAgent generated multiple duplicate AI messages in the conversation history (scratchpad) when the LLM returned multiple tool calls in a single response turn.

The Fix:
The constructScratchPad function groups sequential tool calls into a single AIChatMessage only if they share the same Log string. Previously, ParseOutput generated a unique Log for every tool call (stating only one tool at a time), which caused the grouping logic to fail and create multiple AI message turns.

I refactored ParseOutput to generate a sharedLog string that summarizes all tools being invoked in that response. By making the Log identical for all actions in a choice, constructScratchPad now correctly collapses them into a single message with multiple ToolCalls.

Verification:

• Added a targeted reproduction test case TestConstructScratchPadGrouping in agents/reproduce_1463_test.go (verified that 2 parallel tool calls result in 1 AI message + 2 Tool responses).
• Verified that existing tests in openai_functions_agent_test.go (including TestOpenAIFunctionsAgent_ParseOutput_MultipleToolCalls) still pass.

Fixes #1463