Fix Bedrock Converse input_tokens double-subtracting cache tokens

[jmangel]

Fixes #828.

Problem

Converse::Chat#input_tokens subtracts cacheReadInputTokens and cacheWriteInputTokens from inputTokens. But AWS Bedrock's inputTokens is already the non-cached count. The cache buckets are reported separately, not folded in. Per the AWS prompt-caching docs:

When prompt caching is enabled, the inputTokens field represents only the non-cached input tokens... total input tokens = inputTokens + cacheReadInputTokens + cacheWriteInputTokens

So the subtraction removes tokens that were never there, understating input on every cache hit and flooring it to 0 whenever the cached prefix exceeds the fresh input (the common multi-turn case). Any cost or usage tracking on message.input_tokens under-reports.

Why pass-through is correct

The Anthropic provider already handles the same Claude models correctly: anthropic/chat.rb passes input_tokens through raw and reads the cache buckets separately. Bedrock serves those same models with the same semantics, so this just brings Converse into line. The same fix landed in LiteLLM (#15292) and was reported in LangSmith (#1858).

Change

• input_tokens returns usage['inputTokens'] unchanged. cached_tokens and cache_creation_tokens are still parsed separately, so nothing is lost. Streaming delegates to this method, so both paths are covered.
• Updated the converse chat spec. The previous test asserted the subtraction on a payload that cannot occur under AWS semantics (inputTokens already excludes cache), so its fixture is corrected to a realistic one, and a regression is added for the floor-to-zero case.

Verified: bundle exec rspec spec/ruby_llm/protocols/converse/ plus spec/ruby_llm/providers/bedrock_spec.rb green (39 examples), rubocop clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.17%. Comparing base (5e252ea) to head (bf4bbfb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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- Coverage   82.18%   82.17%   -0.02%     
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  Files         170      170              
  Lines        8029     8027       -2     
  Branches     1347     1347              
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- Hits         6599     6596       -3     
- Misses        890      891       +1     
  Partials      540      540              

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