fix(gemini): prevent parallel tool call slot collisions in native streaming

[cycorld]

Summary

Fixes an issue where Gemini 3.x / 2.5 streaming responses containing multiple parallel tool calls with the same tool name (e.g. 3 parallel search_files calls) collided into slot index 0.

Problem & Root Cause

When Gemini emits parallel tool calls across separate SSE events in streamGenerateContent?alt=sse, each SSE event chunk delivers a single candidate with parts[0] (part_index == 0).

In translate_stream_event(), call_key previously hashed only:

call_key = json.dumps(
    {
        "part_index": part_index,
        "name": name,
        "thought_signature": thought_signature,
    },
    sort_keys=True,
)

When Gemini streamed consecutive parallel tool calls of the same name, call_key was identical across chunks, mapping every tool call to the same slot (index: 0). The streaming accumulator (chat_completion_helpers.py:tool_calls_acc) appended arguments into one string:
{"pattern": "*a*"}{"pattern": "*b*"}{"pattern": "*c*"}

Because the resulting string was invalid concatenated JSON, argument parsing failed and the loop exhausted retries, ultimately misdiagnosing the turn as:
Response truncated due to output length limit

Solution

1. Use functionCall.id in call_key (f"id:{fc_id}") when present (standard in Gemini 3+).
2. For legacy models without IDs (Gemini < 3), include args in the fallback call_key so different calls at part_index == 0 receive distinct slot indices.
3. Added unit test coverage for parallel tool calls across SSE chunks for both Gemini 3+ and legacy models.

Verification

Ran full test suite:

• pytest tests/agent/test_gemini_native_adapter.py (31/31 passed)
• pytest tests/agent/test_gemini_*.py tests/hermes_cli/test_gemini_*.py (78/78 passed)

[alt-glitch]

This was generated by AI during triage.

Duplicate of #75528, which already fixes provider-ID slot identity and the id-less fallback while preserving incremental argument continuations.