fix(bedrock): add prompt-cache pricing to Claude Bedrock entries

[jaketracey]

Problem

The Bedrock Claude pricing entries (added in bedrock-pricing-2026-04) set input_cost_per_million and output_cost_per_million but omit cache rates. As a result, cache_read_tokens and cache_write_tokens are billed at $0 in cost estimates — so a cached agentic session (where the cached system+tools prefix is usually the majority of input) reports a cost far below reality.

Fix

Add the standard Anthropic prompt-cache multipliers to the four Claude Bedrock entries (same rates apply on Bedrock):

• cache read = 0.1× base input
• 5-minute cache write = 1.25× base input

model	input	cache_read	cache_write
claude-opus-4-6	15.00	1.50	18.75
claude-sonnet-4-6	3.00	0.30	3.75
claude-sonnet-4-5	3.00	0.30	3.75
claude-haiku-4-5	0.80	0.08	1.00

Input/output rates are unchanged. (Nova/other Bedrock models also support caching with different multipliers and could be added separately.)

🤖 Generated with Claude Code

[teknium1]

Thanks for the focused pricing correction. Current main already provides this exact behavior.

• agent/usage_pricing.py:517-561 contains the same cache-read and 5-minute cache-write rates for all four Bedrock Claude entries.
• agent/usage_pricing.py:939-965 returns unknown when a required cache rate is absent and otherwise includes both cache buckets in the estimate.
• tests/agent/test_usage_pricing.py:255-324 now verifies both the cache-rate invariant and that a cached Bedrock Claude session produces an estimated cost; this also covers the symptom discussed in related Bedrock Claude /usage shows cost unknown — pricing table lacks cache cost fields + cross-region prefix normalization #50295.
• The implementation is present in d682f320b35a13084371a541a835e1d988c982b8.

Automated hermes-sweeper review.