Merge streaming tool call deltas by index in OpenAiChatModel

[jewoodev]

Summary

Streaming tool calls through OpenAiChatModel against OpenAI-compatible endpoints such as DeepSeek are broken since 2.0.0-RC1: every argument fragment surfaces as a separate ToolCall with an empty id and name, so tool execution receives null or empty arguments.

ChunkMerger.mergeDeltas treats a delta as the start of a new tool call when toolCall.id().isPresent(). OpenAI omits id on continuation deltas, but DeepSeek-style endpoints send "id": "", which is present, so each fragment is appended as a new tool call instead of being concatenated into the previous one. Up to 2.0.0-M8 the aggregation keyed tool call builders by choice.index() + "-" + toolCall.index() and was not affected; the behavior changed when 12f5cbe (#6288) replaced that aggregation, and d72057c later moved the logic into ChunkMerger unchanged.

This change keys the merge on the delta's required index field, which is what the streaming API defines for correlating tool call deltas. It also lifts the previous one-tool-call-per-chunk Assert and merges parallel tool call deltas correctly when fragments interleave across indexes.

Test

ChunkMerger is now package-private (was private) for direct unit testing. The new tests cover OpenAI-style absent continuation ids, DeepSeek-style empty-string ids (the gh-6374 reproduction), parallel tool calls, and multiple tool call deltas in a single chunk. Verified with ./mvnw -pl models/spring-ai-openai package.

Relation to existing work

#6369 fixes the empty-id continuation case narrowly for #4790. This PR takes the index-keyed path in the current 2.0 OpenAiChatModel.ChunkMerger, which also covers omitted continuation ids, empty-string continuation ids, parallel tool-call indexes, and multiple tool-call deltas in one chunk. Older PRs such as #5973/#4794 target the previous helper-layer code path, while this PR is scoped to the current ChunkMerger implementation.

Fixes gh-6374

[jewoodev]

Rebased onto main to resolve a conflict with #6365.

#6365 changed mergeDeltas to preserve tool-call additional properties. This PR replaces that merge logic with by-index merging, so I carried the preservation over: ChunkMerger.mergeToolCalls now copies each delta's additional properties via putAllAdditionalProperties.

The streaming test added in #6365 (preserveUnmappedToolCallAdditionalProperties) verifies this. It fails without that call and passes with it, along with the rest of the chat model tests.