ci: use FIPS image variants in langsmith readonly test - #692

Summary

• Overrides the 7 LangChain-authored image repositories in charts/langsmith/ci/readonly-config-values.yaml to their -fips counterparts so the readonly CI job exercises FIPS images alongside the readOnlyRootFilesystem: true configuration.
• Only repository is overridden — tag and pullPolicy continue to flow from the chart's default values.yaml, so each release automatically pulls the matching FIPS build.
• Follows the image mapping documented in docs: add self-hosted FIPS images guide docs#3712. Third-party images (postgres/redis/clickhouse) and LangChain images without a published FIPS variant (clio, agent-builder-*, polly) are untouched.

Test plan

• helm lint charts/langsmith -f charts/langsmith/ci/readonly-config-values.yaml
• helm template charts/langsmith -f charts/langsmith/ci/readonly-config-values.yaml shows the 7 overridden repositories and default tag
• CI readonly workflow pulls and runs the FIPS images successfully