feat(agenticclaude): expose cache_creation_input_tokens in message.Extra

[sin-bufan]

Summary

Port cache_creation_input_tokens from the legacy claude adapter to agenticclaude, following the pattern established in #929.

Anthropic bills prompt cache reads (~10% of base rate) and writes (~125% of base rate) at very different prices. The read side is already exposed through the standard PromptTokenDetails.CachedTokens path. The creation side (cache_creation_input_tokens) has no standard schema equivalent, so it is surfaced in message.Extra:

• Key: _eino_claude_cache_creation_input_tokens (same key as the legacy claude adapter)
• Set in: toAgenticMessage (Generate) and toMessageStreamingChunk (message_delta)
• Read via: agenticclaude.GetCacheCreationInputTokens(msg *schema.AgenticMessage) (int, bool)
• Streaming: Anthropic reports once per request on message_start or message_delta. int has built-in useLast concat, so ConcatAgenticMessages correctly preserves the value without accumulation.
• Zero activity: When CacheCreationInputTokens == 0, no Extra entry is written.

Design decisions

• Same Extra key as legacy adapter — Downstream consumers (e.g. Langfuse cost tracking) do not need to distinguish between legacy and agentic adapters.
• Only cache_creation in Extra — cache_read is NOT duplicated because it's already in the standard PromptTokenDetails.CachedTokens path.
• int type, not a struct — Simpler than a typed struct; no need to register concat func since int has built-in support.

Changes

File	Change
consts.go	Add exported KeyOfCacheCreationInputTokens constant
convertor.go	Set cache_creation in toAgenticMessage (Generate path)
event_convertor.go	Set cache_creation in toMessageStreamingChunk (message_delta)
message_extra.go	New: GetCacheCreationInputTokens() getter
message_extra_test.go	New: tests for getter, Generate path, streaming delta path

Test plan

• TestGetCacheCreationInputTokens — nil message, nil Extra, key missing, key present
• TestCacheCreationInputTokens_SetInConvOutputMessage — zero and nonzero paths
• TestCacheCreationInputTokens_SetInStreamDelta — zero and nonzero paths
• All existing tests pass (go test ./...)

Related

• feat(claude): expose cache_creation_input_tokens in message.Extra and add Config.AutoCacheControl #929 — Same feature for the legacy claude adapter (already merged)