fix: enable native Anthropic prompt caching

[eliebak]

Direct Anthropic requests currently use Chat Completions, which does not support prompt caching. Route api.anthropic.com through the native Messages API and enable automatic five-minute caching, with a separate system-prompt breakpoint for reuse across compaction.

Add optional api_format, prompt_cache (5m, 1h, or off), and max_output_tokens provider settings. Preserve native content and tool history, handle Anthropic retries, context overflows, and context-window discovery, and count cache writes toward token budgets while excluding cache reads. Full prompt usage still includes both for context management.

Validation:

• uv run pytest tests/: 338 passed; 5 opt-in live tests skipped.
• Live Anthropic tests: 5 passed, covering cache lifetimes, growing history, caching disabled, tool execution, compaction, and ACP stdio follow-ups.
• Markdownlint, Ruff lint, and formatting checks passed.

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Note

Medium Risk
Changes the default inference path for Anthropic endpoints and token-budget accounting; mistakes could affect cost limits or break tool/compaction flows, though behavior is heavily tested.

Overview
Anthropic traffic now uses the native Messages API when provider.base_url is api.anthropic.com (or api_format is "anthropic"), instead of Chat Completions, so prompt caching works. OpenAI-style chat history is converted in a new transport layer; responses are normalized back to ChatCompletion, including anthropic_content for faithful tool replay.

The ACP provider contract gains api_format (auto / openai / anthropic), prompt_cache (5m, 1h, off), and max_output_tokens. Requests set ephemeral cache controls (plus a system-prompt breakpoint for reuse after compaction). Tree max_total_tokens billing treats cache reads as non-new work while still counting full prompt usage for compaction thresholds; Anthropic retries, overflow detection, and model context-window discovery (max_input_tokens) are wired through the shared client path.

Documentation describes caching behavior and opt-in live tests; anthropic is added as a dependency with unit and live test coverage.

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