ADFA-6318 | Describe native tool calling in the AI-Agent-OpenAI README

[hal-eisen-adfa]

Fixes point 2 of ADFA-6318. Point 1 (docs/plugin-api.md in CodeOnTheGo) is a separate PR in that repo.

What was stale

• Native function calling said the backend does not declare ToolCallingBackend. That text dates from ADFA-3017; native tool calling landed in ADFA-5410 (OpenAiBackend implements ToolCallingBackend and overrides generateStreamingWithTools).
• Key classes omitted OpenAiToolProtocol.kt (the tool protocol) and OpenAiHttpClient.kt (the actual HTTP transport, which the list attributed to OpenAiBackend).
• Test count said 177; there are 245 @Test methods under src/test.

Change

README only. The section now describes the request (tools[], tool_choice only for a required tool), stream (tool_calls deltas accumulated, reported after the stream ends), results (sent as user turns), prompt (native format from tools.yml), and the fallback when a server refuses tools (retry once with none, Toast, remembered per base URL). Each claim was checked against the code.

Verification

addons check passes. Docs-only; no build or device run.

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