chore: remove fake_crypto from spec tests once ssv-spec fixtures are fixed

[shane-moore]

Context

PR #942 adds proposer consensus-data spec tests that require fake_crypto to run full assertions. Without it, Lighthouse's SSZ decoder rejects the invalid BLS points in ssv-spec's Capella/Deneb proposer fixtures, and the tests fall back to a path that skips block root, block SSZ, and blinded state checks.

The root cause is fixture quality in ssv-spec: the proposer fixtures use placeholder BLS values (0x000102... pubkeys, 0x606162... signatures) copied from go-eth2-client codec tests. These are correct length but not valid compressed BLS curve points. Go accepts them because its decoder only checks hex+length.

Upstream

• Issue: Capella/Deneb proposer fixtures reuse codec-only BLS placeholder bytes and fail strict decoders ssvlabs/ssv-spec#622
• Fix PR: chore: block proposer fixtures become bls valid ssvlabs/ssv-spec#627 (open, against spec-test-ci)

Action

Trigger: ssv-spec #627 lands and the spec-tests JSONs are regenerated against it; bump the Anchor submodule to that revision. Then:

1. Remove the fake_crypto feature from spec_tests/Cargo.toml
2. Remove the BLS fallback path in ConsensusDataProposerTest (is_bls_validation_error handling in consensus_data_proposer.rs)
3. Remove the BLS fallback path in SSZSpecTest (is_bls_validation_error handling in ssz_spec_test.rs)
4. Remove the BLS fallback in can_decode_block (is_bls_validation_error handling in encoding_helpers.rs)
5. Remove the BLS fallback in ProposerConsensusDataTest (uses can_decode_block in proposer_consensus_data.rs)
6. Remove the is_bls_validation_error helper from encoding_helpers.rs
7. Drop any CI targets that run spec tests with --features fake_crypto

Discussion

• Diego's review: test: proposer consensus-data spec coverage #942 (comment)
• Follow-up: test: proposer consensus-data spec coverage #942 (comment)
• Analysis: test: proposer consensus-data spec coverage #942 (comment)
• Diego on workaround accumulation (PR ci: run spec_tests with fake_crypto #983): ci: run spec_tests with fake_crypto #983 (comment)