GH-5806: Merge streaming tool call chunks in MessageAggregator

[suryateja-g13]

Summary

• Streaming tool calls arrive in multiple chunks: first chunk carries id and name, subsequent chunks carry partial arguments with an empty id
• MessageAggregator was calling addAll() on every chunk, producing a list of incomplete ToolCall entries with empty arguments, ultimately causing IllegalArgumentException: toolInput cannot be null or empty during execution
• Added mergeToolCallChunk() helper: chunks with a non-empty id start a new entry; chunks with an empty id append their arguments to the most recently started call
• Handles single tool calls split across N chunks and multiple sequential tool calls streamed one after another

Test plan

• MessageAggregatorTests#should_merge_tool_call_chunks_into_single_complete_tool_call — replicates the exact streaming pattern from the issue report (4 chunks, first with id+name, rest with empty id and partial arguments)
• MessageAggregatorTests#should_pass_through_complete_tool_call_unchanged — single complete tool call (no merging needed) still works
• MessageAggregatorTests#should_merge_multiple_parallel_tool_calls — two sequential tool calls each split across two chunks produce two distinct merged entries
• MessageAggregatorTests#should_handle_chunk_with_null_arguments — null arguments in first chunk handled without NPE

Fixes GH-5806 (#5806)

🤖 Generated with Claude Code

[anuragg-saxenaa]

LGTM. The fix changes addAll to a mergeToolCallChunk() loop that correctly handles partial tool call chunks: chunks with a non-empty id start a new entry, while empty-id chunks append arguments to the last entry. The logic correctly reconstructs streamed tool calls across multiple chunks.

The new MessageAggregatorTests has good coverage: merging sequential chunks, passing through complete tool calls, handling multiple parallel tool calls, and null arguments. Edge case handling looks solid.

One minor observation: the code uses StringUtils.hasText(chunk.id()) for the empty-id check. Since id is a String record component, this is safe — empty string or null both return false from hasText. ✅