fix(openai): recover final response tool arguments

[Sayt-0]

Summary

• recover Responses API function arguments when providers omit response.function_call_arguments.delta
• use response.function_call_arguments.done and the completed output-item snapshot as non-duplicating fallbacks
• buffer final arguments received before response.output_item.added and ignore stale late deltas
• add regression coverage for shell/read-file argument shapes, interleaved calls, reordered events, and duplicate prevention

Issue expectations

Expectation	Implementation
shell receives its cmd argument with GitHub Copilot OpenAI models	Final arguments are emitted from response.function_call_arguments.done when no deltas were delivered
read_file receives its path argument	The same provider-level recovery applies to every Responses API function tool
No tool-call doom loop	Empty arguments are no longer forwarded, and final snapshots are emitted only once
Preserve normal Responses API streaming	Existing argument deltas remain unchanged; final events only provide fallbacks and terminate stale deltas

Validation

• deterministic regression reproduced before the fix with empty arguments, then passed after the fix
• live GitHub Copilot checks passed with gpt-5.5 and gpt-5.6 variants for shell and read_file
• go test ./pkg/model/provider/openai -run 'TestResponseStream_' -count=20
• go test -race ./pkg/model/provider/openai
• task test
• task lint
• task build

Residual uncertainty

GitHub Copilot may expose undocumented stream orderings beyond those observed. The implementation covers the standard ordering, missing argument deltas, final snapshots, early final events, interleaved calls, and stale late deltas.

Fixes #3818