fix(proxy): ignore empty legacy function_call placeholders

[u1544096979]

Summary

• ignore empty legacy function_call placeholders during OpenAI Chat to Anthropic SSE conversion
• preserve existing handling when either the legacy function name or arguments contains data
• add a regression test for the terminal empty-object case

Tests

• cargo test --manifest-path src-tauri/Cargo.toml empty_legacy_function_call_does_not_emit_tool_use
• cargo test --manifest-path src-tauri/Cargo.toml legacy_function_call_stream_emits_tool_use_block_and_argument_delta
• cargo test --manifest-path src-tauri/Cargo.toml proxy::providers::streaming::tests (21 passed)
• live /v1/messages stream against the affected OpenAI-compatible provider: text, end_turn, and message_stop preserved; empty tool_use count is zero

Closes #437

[SaladDay]

Please run cargo fmt from src-tauri/ and push the changes to fix the failing formatting check.