refactor(api): rename attestation mode to TEE variant

[kvinwang]

Summary

• rename KMS Auth API attestationMode to teeVariant
• rename Onboard.GetAttestationInfo.attestation_mode to tee_variant while preserving protobuf field number 4
• rename verifier details.attestation_mode and policy boot info to tee_variant / teeVariant
• update the onboarding UI, auth-simple schema/tests, runbooks, and API documentation

Compatibility

This intentionally changes the JSON/source API names. The protobuf field number remains 4, so protobuf binary wire compatibility is preserved for GetAttestationInfo.

Validation

• cargo check --workspace
• cargo test -p dstack-kms -p dstack-verifier -p dstack-attest
• strict Clippy for the affected Rust crates

bun run check was not run because Bun is not installed in the development environment.

Stack

Stacked on #806; merge that PR first.

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