fix(bedrock): emit prompt-caching counter as gen_ai.prompt_caching

[sayonfortify]

Summary

• Change the Bedrock prompt-caching counter name from gen_ai.prompt.caching to the canonical contract name gen_ai.prompt_caching.
• Add a deterministic test asserting the counter name matches CacheSpanAttrs.CACHED and the backend/catalog contract spelling.

Why

Prometheus normalizes both spellings to gen_ai_prompt_caching, so current Mimir visibility works. But the OTel/contract name should match exactly for registry/discovery, docs, and SDK/backend round-trips.

Validation

• python3 -m py_compile packages/opentelemetry-instrumentation-bedrock/opentelemetry/instrumentation/bedrock/__init__.py packages/opentelemetry-instrumentation-bedrock/tests/metrics/test_bedrock_prompt_caching_metrics.py
• AST assertion over actual source:
  • PromptCaching.LLM_BEDROCK_PROMPT_CACHING == "gen_ai.prompt_caching"
  • CacheSpanAttrs.CACHED == "gen_ai.prompt_caching"
• git diff --check

Local limitation:

• pytest packages/opentelemetry-instrumentation-bedrock/tests/metrics/test_bedrock_prompt_caching_metrics.py -q is blocked in this checkout by missing boto3 from the package test environment. CI/fork env should run this file with the package dependencies installed.

Vendoring sequence

Per internal-sop/openllmetry/VENDORING_SOP.md, do not PR the ocelle-py vendored copy directly from this untagged branch. After this merges:

1. Tag the fork as fr-v0.52.6.22.
2. Run ocelle-py/scripts/vendor.sh ../fr-openllmetry-py . fr-v0.52.6.22.
3. Review src/fortifyroot/_vendor/ and VENDOR_MANIFEST.json.
4. Open the ocelle-py vendoring PR.

No PyPI/RC release is included here.