fix(loop): finalize bounded native tool turns safely

[Brian Krabach (bkrabach)]

What changed

Completes the loop-streaming downstream needed by the merged provider boundary fixes:

• OpenAI PR #89: fix: preserve portable tool choice for native computer requests amplifier-module-provider-openai#89 (merged as 88b8c07a6940f74c0ef17d45258dd5ea35b1bd6b)
• Anthropic PR #116: fix: translate portable tool choices at the Anthropic boundary amplifier-module-provider-anthropic#116 (merged as 4cf78c792dd214233d16b373e824135de5687957)

The loop now distinguishes a natural completion at the iteration boundary from a forced finalization. Natural completion does not make a duplicate provider call. Forced finalization preserves the ordinary generic/native tool declarations while sending portable tool_choice="none"; unexpected tool calls are not dispatched or structurally persisted. Cancellation and provider errors close an interrupted tool turn safely, while reasoning blocks, provider metadata, and llm_calls remain accurate.

No provider model names are hardcoded here; provider dialect behavior remains at the provider boundary.

Type of change

• Bug fix
• New feature
• Documentation
• Refactor / internal change (no behavior change)
• Other:

Testing

• Full local suite: 266 passed.
• Boundary coverage includes natural completion versus forced finalization, the final request's tool_choice="none", non-dispatch of an unexpected response tool call, cancellation/error closure, native tool-spec retention, reasoning metadata, and accurate provider-call accounting.
• Provider prerequisites were independently merged with green CI: OpenAI PR #89 (py3.11, py3.12, CLA) and Anthropic PR #116 (Ubuntu/macOS/Windows on py3.11 and py3.12, CLA).

Review focus

• The normal-completion/forced-finalization boundary must not regress into an extra call for a complete response.
• The finalization request must retain native declarations for transcript validity while preventing another tool dispatch.
• The provider merge links above are dependencies/evidence only; this PR does not request or perform an admin merge.