fix(ai): normalize Bedrock cache usage

[opencode-agent]

Summary

• treat Bedrock Converse inputTokens as non-cached input, matching AWS prompt-caching semantics
• derive normalized inclusive input usage by summing non-cached, cache-read, and cache-write tokens
• update usage contract documentation and cover the mapping with focused synthetic and recorded event-stream tests
• record a real bearer-authenticated cache write followed by a cache read, asserting the normalized usage invariant on replay

AWS reference: https://docs.aws.amazon.com/bedrock/latest/userguide/prompt-caching.html

Checks

• bun test test/provider/bedrock-converse.test.ts test/provider/bedrock-converse-cache.recorded.test.ts (34 passed)
• bun typecheck
• bun x prettier --check packages/ai/src/protocols/bedrock-converse.ts packages/ai/src/schema/events.ts packages/ai/test/provider/bedrock-converse.test.ts packages/ai/test/provider/bedrock-converse-cache.recorded.test.ts
• cassette verified to contain neither the authorization header nor bearer token

Requested by: @rekram1-node (Aiden Cline via Slack)