Use TAV for SNP attestation verification

[cjen1-msft]

Integrates TAV into CCF for snp attestation verification.

This might be our first 8.X PR as the 'optimal' deduplication approach will be that TavSnpAttestation replaces AttestationReport, but that then turns the accessors from data accessors into a struct into ffi calls.
So this would be a breaking API change.

For purely internal use this would not be a breaking change.

Update:
Specifically for https://github.com/microsoft/scitt-ccf-ledger this would break their current build.
I think the best option here is to mark the PODS struct as deprecated and move all our internal stuff over to TAV.
Then in either 8.X or 9.X we dedupe it.

[Copilot]

Pull request overview

This PR integrates the TEE Attestation Verification (TAV) FFI crate into CCF’s Rust umbrella (ccf-rs) and switches SEV-SNP attestation verification in the PAL layer to use tav_verify_snp_attestation, while keeping CCF’s existing report parsing and policy checks.

Changes:

• Add the TAV FFI crate to ccf-rs and update the locked Rust dependency graph.
• Replace PAL-side SNP certificate-chain, signature, and TCB verification logic with a TAV verification call and map failures to CCF errors.
• Update SEV-SNP attestation validation tests and build system wiring (Rust build deps, PAL include/link settings).

Custom instructions used:

• .github/copilot-instructions.md
• .github/instructions/reviewing.instructions.md

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/rust/src/lib.rs	Re-exports the new TAV FFI crate from the ccf-rs umbrella.
src/rust/Cargo.toml	Adds the vendored tee-attestation-verification-ffi dependency with OpenSSL crypto feature.
src/rust/Cargo.lock	Locks new transitive dependencies introduced by TAV integration.
src/pal/test/snp_attestation_validation.cpp	Adds/updates negative tests to expect TAV failure paths and error codes.
src/pal/tav_ffi.h	Introduces RAII wrappers for TAV error/report handles used by PAL.
src/pal/attestation.cpp	Replaces local SNP chain/signature verification with tav_verify_snp_attestation and consistent error reporting.
CMakeLists.txt	Updates ccf_pal link libraries and adds include path for TAV FFI headers.
cmake/ccf_rs.cmake	Ensures the Rust build target reruns when the vendored TAV FFI Cargo.toml changes.