Parse function_call-wrapped and bare JSON tool calls

[chubes4]

Summary

WP_Agent_Provider_Turn_Result's text tool-call fallback dropped tool calls in two shapes that real providers emit, so the model's tool call silently never executed:

1. Bare JSON object — a tool call emitted as the entire message with no <tool_call> tag and no ```json fence. extract_json_tool_calls() only collected payloads from tags/fences, so bare JSON was never parsed.
2. function_call wrapper — {"type":"function_call","function_call":{"name":...,"arguments":{...}}}. The parser read name/function/arguments at the top level and never unwrapped the inner function_call object, yielding an empty call.

Both produced zero extracted tool calls.

Changes

• extract_json_tool_calls(): when no tag/fence payloads are found, treat a trimmed message that is a single JSON object ({...}) as a payload candidate.
• tool_calls_from_json_payloads(): unwrap an inner function_call object before reading the tool name and arguments.
• Added smoke coverage in tests/provider-turn-adapter-smoke.php for the function_call wrapper shape.

Testing

• php tests/provider-turn-adapter-smoke.php → all 49 assertions pass (2 new).
• Full agents-api smoke suite: all 80 tests/*-smoke.php files pass by exit code.
• Verified end-to-end against a live Codebox coding task (Claude Opus 4.8) — results posted on Add Claude Code provider plugin Extra-Chill/wp-coding-agents#197.

Context

Discovered while landing the ai-provider-for-claude-code carried provider (Extra-Chill/wp-coding-agents#197) and the writable-workspace-tools fix (Extra-Chill/homeboy-extensions#1190). With those in place, Claude Opus 4.8 correctly emitted a workspace_write tool call, but it was dropped here because of the unrecognized shape. This is the final link making real Codebox coding tasks work.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Root-caused the dropped tool-call shapes from live transcripts, implemented the parser fix, added smoke coverage, and ran the full suite plus the end-to-end canary; Chris directed the work and remains responsible for review/testing.

[chubes4]

Closing without merge.

This PR widened the text tool-call scraper to absorb more malformed shapes (bare JSON objects and {"type":"function_call","function_call":{...}} wrappers). That is a paper-over: the canonical contract is that provider plugins return structured tool calls via wp-ai-client FunctionCall parts, which extract_tool_calls() resolves at extract_structured_tool_calls() and returns early — before any text scraping runs.

The real fix landed in the Claude Code provider (Extra-Chill/wp-coding-agents#197), which now declares Anthropic tools and parses tool_use blocks into structured FunctionCall parts. With that, the text fallback is never reached for the provider that motivated this change, so growing the scraper here is unnecessary and would calcify acceptance of non-canonical provider output.

If we want agents-api to actively enforce the contract, the better follow-up is a _doing_it_wrong() warning when a provider only yields tool calls via text scraping — tracked separately rather than by extending the scraper.