[STACKED on #964] fix(verifier): validate one-shot certificate profile

[kvinwang]

STACKED PR — merge #964 first.

Problem

The dstack-verifier --verify-cert one-shot path reads a certificate directly from disk, so it does not pass through rustls/WebPKI certificate validation. Certificates with an invalid self-signature, validity period, key usage, extended key usage, or SAN profile could reach the one-shot attestation trust decision.

Root cause and fix

Validate the required endpoint certificate profile in dstack-verifier before performing RA-TLS attestation and public-key-binding verification.

The profile policy intentionally lives in dstack-verifier, not the ra-tls library: ra-tls remains responsible only for parsing and cryptographically verifying the embedded attestation and its binding to the certificate SPKI. This avoids implying that the reusable library API performs general TLS/WebPKI endpoint validation.

Implementation

• Validate the self-signed endpoint certificate signature and validity period.
• Require digital-signature key usage.
• Require server-auth or client-auth extended key usage.
• Require a non-empty SAN extension.
• Normalize PEM input to DER before applying the one-shot policy and RA-TLS verification.
• Enable x509-parser signature verification only in dstack-verifier.

Changed paths relative to the stacked parent:

• dstack/Cargo.lock
• dstack/verifier/Cargo.toml
• dstack/verifier/src/main.rs

Scope

This PR addresses the certificate-file one-shot path only. Normal TLS connections remain subject to their TLS stack's certificate verification. The reusable ra-tls verification API continues to verify attestation evidence and SPKI binding without imposing an application-specific X.509 profile.

Dependency and merge order

• Parent PR: [STACKED on #880] feat(simulator): add deterministic seeded attestations #964
• GitHub base branch: codex/feat-simulator-seeded-attestation
• Merge the parent first; this PR contains only the additional delta above that parent.

Verification

• cargo check -p dstack-verifier: passed.
• cargo test -p ra-tls --lib: passed (13 tests).
• git diff --check: passed.

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