feat(anthropic): handle pause_turn and refusal stop reasons

[adamthehutt]

Summary

Anthropic returns stop_reason="pause_turn" when a long-running server-side tool (web_search, web_fetch, code_execution, MCP connectors, etc.) needs the client to continue the turn, and stop_reason="refusal" when the model declines to respond. Today both fall through the Text handler's match and surface as a generic:

Anthropic: unknown finish reason

…with no information about which stop reason actually occurred. For requests using web_search / web_fetch provider tools this can fail an entire job after several minutes of work, with no way to recover or even diagnose what happened.

Changes

• Add FinishReason::Pause and FinishReason::Refusal cases.
• Map pause_turn and refusal in Anthropic\Maps\FinishReasonMap.
• Implement pause_turn resume in Anthropic\Handlers\Text per Anthropic's documented protocol: append the assistant message to the conversation and re-send the request so the model can continue. Bounded by the existing maxSteps() guard, identical in shape to how handleToolCalls() already loops.
• Throw a descriptive PrismException for refusal that names the stop reason.
• Surface the raw stop_reason string in the fallback "unhandled finish reason" exception, so any future stop reason that Anthropic introduces is debuggable without patching the library.
• Add fixture-backed tests covering:
  • The pause_turn resume flow producing a final response with two steps.
  • The pause_turn resume flow respecting maxSteps() and stopping early.
  • The refusal exception path.

Out of scope

The Stream handler is not modified. Streaming pause_turn resume is more involved (the partial assistant message needs to be reassembled from deltas before being re-sent) and is best handled in a follow-up PR. Today, streaming pause_turn will still fall through to the existing FinishReason::Stop default — which is no worse than current behaviour.

Test plan

• vendor/bin/pest tests/Providers/Anthropic/ — 155 tests pass (3 new, 152 existing).
• New tests:
  • pause_turn → it resumes the turn when Anthropic returns stop_reason="pause_turn"
  • pause_turn → it stops resuming once maxSteps is reached
  • exceptions → it throws a descriptive exception when Anthropic returns a refusal stop_reason

References

• Anthropic docs on pause_turn: https://docs.anthropic.com/en/api/handling-stop-reasons#pause-turn
• Anthropic docs on refusal: https://docs.anthropic.com/en/api/handling-stop-reasons#refusal