feat: enable Anthropic prompt caching via automatic cache_control injection (#660)

* feat(llm): add Anthropic prompt caching and cache token tracking

- Inject cache_control via additional_params for Claude models in rig_adapter
- Add cache_read_input_tokens and cache_creation_input_tokens to
  CompletionResponse and ToolCompletionResponse
- Extract cached_input_tokens from rig-core unified Usage
- Add is_anthropic_model() detection helper with provider prefix support
- Log prompt cache hits at debug level (consistent with response_cache)
- Add 7 unit tests for cache injection and model detection
- Update all mock providers and test fixtures with new fields

* feat(cost): apply 90% cache discount to prompt-cached tokens in CostGuard

- Add cache_read_input_tokens to TokenUsage so cache counts flow from
  CompletionResponse through the reasoning layer to the dispatcher
- Update CostGuard::record_llm_call() to accept cache_read_input_tokens:
  cached tokens are billed at 10% of the normal input rate
- Thread cache_read_input_tokens from dispatcher into CostGuard
- Add test_cache_discount_reduces_cost verifying exact savings match
  90% of input cost for fully-cached requests
- Update all existing test callers with zero-cache parameter

* refactor(cache): scope cache_control to Anthropic backend and validate model support

- Replace model-name-based is_anthropic_model() with explicit
  enable_prompt_cache flag on RigAdapter, set only for the direct
  Anthropic backend via with_prompt_cache(true)
- Add supports_prompt_cache() to validate model names per Anthropic
  docs: only Claude 3+ models support caching; claude-2 and
  claude-instant are excluded to prevent 400 errors
- Warn when caching is enabled but model does not support it
- Replace is_anthropic_model tests with flag-based and model
  validation tests

* fix(cache): validate model at construction and propagate cache metrics through proxy

- Move supports_prompt_cache() check into with_prompt_cache() so
  unsupported models are detected once at construction, not per request
- Add cache_read_input_tokens and cache_creation_input_tokens to
  ProxyCompletionResponse and ProxyToolCompletionResponse with
  serde(default) for backward compatibility
- Pass cache metrics through orchestrator proxy instead of zeroing
- Use claude-opus-4-6 in cache discount test to match Anthropic
  semantics

* feat(llm): add configurable cache retention with write surcharge

- Add CacheRetention enum (none/short/long) to AnthropicDirectConfig
- Parse ANTHROPIC_CACHE_RETENTION env var (default: short)
- Inject TTL-aware cache_control (short=5m ephemeral, long=1h)
- Extract cache_creation_input_tokens from raw Anthropic response
- Add cache_write_multiplier() to LlmProvider trait (1.25x short, 2.0x long)
- Pipe dynamic write multiplier through dispatcher to CostGuard
- Add TokenUsage.cache_creation_input_tokens field
- Add tests for Long TTL injection, 5m and 1h write surcharges
- Document ANTHROPIC_CACHE_RETENTION in .env.example

* docs: fix stale cache_retention field comment

* fix: resolve CI failures after upstream merge

- Add missing cost_per_token arg to cache test callsites
- Apply cargo fmt to long lines in tests and tracing macros

* fix: address Copilot review feedback

- Use saturating_add for cache token sum to prevent u32 overflow
- Tighten supports_prompt_cache to explicitly match claude-3+/claude-4+
  and named families (claude-sonnet/claude-opus/claude-haiku)

* fix: adapt prompt caching to registry architecture and add missing cache fields

- Resolve merge conflicts: adapt CacheRetention and cache injection to
  the declarative provider registry (RegistryProviderConfig replaces
  AnthropicDirectConfig)
- Parse ANTHROPIC_CACHE_RETENTION env var in create_anthropic_from_registry()
- Use Anthropic automatic caching via top-level cache_control in
  additional_params (rig-core #[serde(flatten)] places it at request root)
- Add cache_read/creation_input_tokens fields to all mock LlmProviders
  added on main after PR #291 branched (response_cache, dispatcher,
  provider_chaos, trace_llm)
- Suppress clippy::too_many_arguments on record_llm_call and
  build_rig_request
- Add regression tests for cache injection (short/long/none) and
  cache_write_multiplier values

Co-Authored-By: Canvinus <[email protected]>

* fix: delegate cache_write_multiplier through provider wrappers and make cache_read_discount configurable

The 6 decorator providers (Retry, CircuitBreaker, Failover, SmartRouting,
CachedProvider, RecordingLlm) did not delegate cache_write_multiplier()
to their inner provider, causing it to always return 1.0 instead of the
actual 1.25x/2.0x from RigAdapter. This fix adds delegation for both
cache_write_multiplier() and the new cache_read_discount() method.

Also makes the cache read discount per-provider instead of hardcoding
Anthropic's 90% discount (÷10). OpenAI uses 50% (÷2), so the discount
is now returned by each provider via the LlmProvider trait.

Addresses review feedback on PR #660.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* style: cargo fmt

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* test: add CacheRetention FromStr/Display unit tests

Tests cover primary values, aliases (off/disabled/5m/ephemeral/1h),
case-insensitivity, invalid input error, and Display round-trip.

Addresses Copilot review feedback on PR #660.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Andrey <[email protected]>
Co-authored-by: Andrey Gruzdev <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>

src/orchestrator/api.rs

@@ -160,6 +160,8 @@ async fn llm_complete(
         input_tokens: resp.input_tokens,
         output_tokens: resp.output_tokens,
         finish_reason: format_finish_reason(resp.finish_reason),
+        cache_read_input_tokens: resp.cache_read_input_tokens,
+        cache_creation_input_tokens: resp.cache_creation_input_tokens,
     }))
 }
 
@@ -189,6 +191,8 @@ async fn llm_complete_with_tools(
         input_tokens: resp.input_tokens,
         output_tokens: resp.output_tokens,
         finish_reason: format_finish_reason(resp.finish_reason),
+        cache_read_input_tokens: resp.cache_read_input_tokens,
+        cache_creation_input_tokens: resp.cache_creation_input_tokens,
     }))
 }