fix(sight): guard tool_call name against empty-string overwrite in SSE delta merge

[jfeng18]

Summary

• Mirror the existing id empty-string guard onto name in extract_parts_from_sse_body SSE tool_call delta merging
• Add a discriminating regression test reproducing the DashScope/Qwen continuation-chunk pattern

Problem

DashScope/Qwen-compatible gateways resend function.name as an explicit empty string ("") on continuation SSE chunks, rather than omitting the key. The id field already had an !is_empty() guard (added in #1133), but name did not — so a later chunk's name:"" clobbers the real tool name captured on the first chunk.

This causes is_bash_tool("") to return false for every Bash tool_call arriving via an OpenAI-compatible streaming backend, silently hiding it from the correlator and downstream ToolUse consumers.

Evidence

• Raw SSE capture from DashScope qwen3-30b-a3b-instruct-2507 (Spring AI issue #4790) shows explicit "name":"" on every continuation chunk
• Independent fix for the same DashScope quirk in tinyhumansai/openhuman PR #3610
• Reproduced locally: ECS cosh session via DashScope produced name:"" in genai_events for all tool_calls

Test plan

• New test test_extract_parts_from_sse_body_tool_calls_empty_name_does_not_overwrite — fails without fix, passes with fix
• Existing test_extract_parts_from_sse_body_tool_calls (name-omitted pattern) unaffected
• cargo +1.89.0 test --lib — 973 passed
• cargo +1.89.0 fmt --check — clean
• cargo +1.89.0 clippy --all-targets -- -D warnings — clean
• ECS E2E: run agentsight trace + cosh via DashScope, verify tool_call name is no longer empty in genai_events

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