fix: sensitive metadata suppression + missing tests + doc template fix

[Angel-4903]

Summary

Problems fixed

1. metadata.values emitted "" when suppress_metadata_values is enabled — not valid JSON, breaks jsondecode(). Fixed to emit "{}".
2. Infinite recompute loop — stateMetadataNeedsSuppression treated "{}" as populated. Fixed by treating "{}" as the suppressed sentinel.
3. Doc template out of sync — new experiment bullets were in docs/index.md but not in templates/index.md.tmpl. Fixed.
4. Missing unit tests — rewrote 3 commented-out TestCloakSetValues* tests and added TestDeepCloneMap, TestExtractSensitiveValues, TestStateMetadataNeedsSuppression_*.

[hashicorp-cla-app]

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[hashicorp-cla-app]

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• Angel Raphael

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