fix(adk-middleware): emit REASONING_ENCRYPTED_VALUE for function_call thought signatures

[ferponse]

What & why

Fixes #2072.

The ADK middleware never emits REASONING_ENCRYPTED_VALUE, even with Gemini thinking enabled, because it only reads thought_signature from thought-text parts. In practice Gemini attaches the signature to the function_call part (the encrypted chain-of-thought that led to the tool call), so the encrypted reasoning was silently dropped.

Verified against Gemini 2.5 Flash on Vertex AI (ag-ui-adk 0.7.0 / google-adk 2.3.0), inspecting every part of a run with two tool calls:

part kind=text          thought=True  thought_signature=None
part kind=function_call thought=None  thought_signature=446 bytes   ← signature HERE
part kind=function_call thought=None  thought_signature=336 bytes
part kind=text          thought=None  thought_signature=385 bytes

REASONING_ENCRYPTED_VALUE count across 3 runs before this change: 0.

Change

• Add EventTranslator._translate_function_call_signatures(): reads thought_signature from function_call parts and emits ReasoningEncryptedValueEvent(subtype="tool-call", entity_id=<toolCallId>, encrypted_value=base64(sig)), mirroring the existing subtype="message" path for thought-text parts.
• Called from the non-partial function-call branch in translate(), covering both LRO and non-LRO calls in the event.
• Deduplicated per tool call id (_emitted_signature_tool_call_ids) so partial/non-partial replays of the same call don't emit it twice.
• Guards: only bytes-like signatures and string tool-call ids are emitted (robust against non-signature parts).

The AG-UI spec already defines ReasoningEncryptedValueEvent with subtype: "tool-call" for exactly this case.

Tests

• Two focused tests added to test_event_translator_comprehensive.py:
  • a function_call part with a thought_signature → one REASONING_ENCRYPTED_VALUE with subtype="tool-call", correct entity_id and base64 payload;
  • a function_call part without a signature → none emitted.
• Full middleware suite (integrations/adk-middleware/python): 920 passed, 0 failed.

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[ferponse]

@contextablemark could you sanity-check this fix — emitting REASONING_ENCRYPTED_VALUE for thought signatures that live on function_call parts (issue #2072)? Want to confirm it matches your understanding.

[ferponse]

Thanks @contextablemark! Added self._emitted_signature_tool_call_ids.clear() to reset() right after _predictive_state_tool_call_ids.clear(), as suggested. Pushed in ff892cb.