fix(gemini): stop parallel tool-call args gluing when thought signatures attach (#102787)

[lEWFkRAD]

Problem

On the native Gemini streaming path, parallel tool calls arrive glued together: one call's arguments is {...}{...} (two JSON objects concatenated), which fails to parse and breaks the turn. Reproduced deterministically against origin/main — json.decoder.JSONDecodeError: Extra data: line 1 column 19.

Root cause

translate_stream_event keys its per-call accumulator slots with json.dumps({"part_index", "name", "thought_signature"}). Gemini delivers a functionCall part first without a thoughtSignature, then re-delivers the same part once the turn's thought signatures are attached. When that happens the key changes mid-stream ("thought_signature": "" → the real signature), so the second delivery does not find the existing slot: it opens a phantom second slot at a new index and re-emits the full arguments. The consumer (chat_completion_helpers, which appends streamed arguments fragments per call index) therefore appends the complete args a second time onto the first call's slot — producing {"a":1}{"b":2} on one call and a spurious duplicate on the other.

Fix

Make the slot key stable for the life of one call: (part_index, name) only. The signature is excluded from the key and still rides to the consumer via extra_content (unchanged), so round-tripping signatures back to Gemini on the next request is unaffected.

TDD

• New regression test test_stream_signature_attach_keeps_parallel_call_slots streams the exact issue shape (two read_file calls, signature attached on the second delivery) and asserts every slot parses as single JSON.
• RED on origin/main (6327930): 1 failed — json.decoder.JSONDecodeError: Extra data: line 1 column 19 (char 18)
• GREEN on this branch: 1 passed; full file 29 passed, 1 deselected (the deselected test_async_native_client_... fails identically on clean origin/main — pre-existing event-loop flake, unrelated).
• Adjacent lanes: tests/run_agent/test_repair_tool_call_arguments.py, test_streaming_tool_call_repair.py, tests/agent/test_gemini_schema.py → 14 passed.

Linked issue

Closes #102787

[alt-glitch]

This was generated by AI during triage.

Duplicate of #88149 — same one-line change (drop thought_signature from the translate_stream_event slot key so identity is (part_index, name)), which already superseded #28438 / #15739. The #102787-shaped regression test here is the useful delta; consider contributing it to #88149. Related same-code-site but different-mechanism fixes: #24676, #75528, #99273 (same-name parallel-call collision).