feat: add portable request controls and validation

[jkyberneees]

Summary

• add portable TopP and Stop controls across OpenAI, Gemini, and Anthropic
• complete documented Thinking: "max" behavior and honor explicit budgets across active presets
• reject unsigned Anthropic thinking replay before any network request
• safely encode opaque Anthropic and Gemini model-pagination tokens
• return typed configuration errors for nil requests
• remove deprecated dialer configuration, correct learn-once documentation, and add explicit test timeouts

Verification

• RED-first cross-format request, thinking, replay-validation, and pagination tests added
• make quality passes
• go test -race -count=1 -timeout 120s ./... passes
• make lint passes with zero issues
• statement coverage: 97.8%