fix(attestation): reject malformed certificate data safely

[Alicepoltora]

Summary

The attestation endpoint already validates the outer DCAP quote, but the nested QE/PCK certificate data was still parsed with unchecked unwrap, expect, assert, and direct indexing. A malformed quote could therefore panic the enclave while extracting the FMSPC and issuer before PCCS verification.

This change:

• makes PCK/FMSPC extraction fallible and propagates errors through PCCS collateral lookup;
• validates the minimum nested QE report length, certificate-chain presence, SGX extension encoding, FMSPC length, and trailing bytes;
• keeps a panic boundary around the current infallible dcap-rs nested parser as defense in depth;
• adds regression tests for truncated QE data and malicious nested length fields.

Impact

Malformed attestation evidence now returns a JSON-RPC error instead of terminating the enclave server. Valid certificate extraction and PCCS verification behavior is unchanged.

Verification

• cargo fmt --all --check
• cargo test -p seismic-enclave-server --lib (10 passed)

The full package test command compiles successfully; its privileged bootstrap integration test is skipped/fails in this environment because it requires sudo.