feat: Parallel Function calling and concurrent tool execution

[praneeth999]

Add the parallel_tool_calls parameter to the backends and enable concurrent tool execution.

Closes MAS-80

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# Paste the output of running pre-commit on your changed files:
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Test CLI Command

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 uv run python -m massgen.cli --config massgen/configs/tools/custom_tools/gpt5_nano_custom_tool_with_mcp_parallel.yaml "whats the sum of 123 and 456? and whats the weather of Tokyo and london?"

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[ncrispino]

Can you check this works with #530? See https://ai.google.dev/gemini-api/docs/thought-signatures, they have a section on parallel tool calling.

[praneeth999]

Will look into it.