Fix ChunkMerger streaming: NoSuchElementException, empty id, reasoning_content

[freshman-wang]

Problem

Three bugs in ChunkMerger affect streaming tool_calls with OpenAI-compatible providers (DashScope/Qwen, DeepSeek):

• NoSuchElementException: Optional.get() without null checks on tool call fields
• Empty id causes arguments loss: DashScope sends id="" in continuation chunks, Optional.of("") passes isPresent(), so fragments are never merged
• reasoning_content lost: chunkToChatCompletion only copies content + refusal, drops provider-specific fields

Fix

1. Filter empty string id in mergeDeltas
2. Guard empty tcs1 + function().orElseGet()
3. Carry delta._additionalProperties() to ChatCompletionMessage
4. Replace Optional.get() with orElse("") / ifPresent()

Fixes #6484

[jewoodev]

For review context, this overlaps with #6373 and #6381. #6373 covers the reasoning_content loss, including final advisor/chat-memory aggregation; #6381 covers empty-id tool-call deltas by merging by index with tests. This PR combines parts of both, but keeps the one-tool-call-per-chunk assumption and does not add coverage.

[freshman-wang]

Closing in favor of #6373 (reasoning_content) and #6381 (empty-id tool-call deltas).

After reviewing @jewoodev's comment, I realize this PR overlaps significantly with both. However, there are two NoSuchElementException cases that I encountered in production with DashScope/Qwen that are not covered by either:

1. mergeDeltas: lastFromTc1.function().get() throws when function is absent — my fix used .orElseGet(() -> Function.builder().build()).
2. chunkToChatCompletion: tc.id().get(), tc.function().get(), tc.function().get().name().get(), tc.function().get().arguments().get() all throw without isPresent() checks — my fix used orElse("") / ifPresent().

It would be great if #6381 could also cover these Optional.get() calls. Thanks!