verifier: report attestation mode

[kvinwang]

Summary

• rename verifier output details.tee_platform to details.attestation_mode
• report the serde representation of AttestationMode directly (for example dstack-tdx, dstack-gcp-tdx, dstack-nitro-enclave, dstack-amd-sev-snp)
• update verifier tests and README references

Testing

• cargo fmt --check
• cargo check -p dstack-verifier
• cargo test -p dstack-verifier

[Copilot]

Pull request overview

This PR updates the verifier’s response schema to report the attestation mode (as the AttestationMode serde string) rather than a custom “TEE platform” string, aligning the output with the underlying attestation types.

Changes:

• Rename verifier output field from details.tee_platform to details.attestation_mode.
• Populate attestation_mode directly from the verified quote’s mode() (e.g., dstack-tdx, dstack-gcp-tdx, dstack-nitro-enclave, dstack-amd-sev-snp).
• Update verifier tests and README documentation to reflect the new field and values.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
verifier/src/verification.rs	Removes custom platform-name mapping and sets details.attestation_mode from the verified quote mode; updates related assertions in tests.
verifier/src/types.rs	Changes VerificationDetails schema from tee_platform: Option<String> to attestation_mode: Option<AttestationMode>.
verifier/README.md	Updates example output and field documentation to use attestation_mode and the AttestationMode serde strings.

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