Fix light client merkle proofs (#7007)

Fix a regression introduced in this PR:

- https://github.com/sigp/lighthouse/pull/6361

We were indexing into the `MerkleTree` with raw generalized indices, which was incorrect and triggering `debug_assert` failures, as described here:

- https://github.com/sigp/lighthouse/issues/7005


  - Convert `generalized_index` to the correct leaf index prior to proof generation.
- Add sanity checks on indices used in `BeaconState::generate_proof`.
- Remove debug asserts from `MerkleTree::generate_proof` in favour of actual errors. This would have caught the bug earlier.
- Refactor the EF tests so that the merkle validity tests are actually run. They were misconfigured in a way that resulted in them running silently with 0 test cases, and the `check_all_files_accessed.py` script still had an ignore that covered the test files, so this omission wasn't detected.

testing/ef_tests/check_all_files_accessed.py

@@ -27,10 +27,8 @@ excluded_paths = [
     "tests/.*/.*/ssz_static/PowBlock/",
     # We no longer implement merge logic.
     "tests/.*/bellatrix/fork_choice/on_merge_block",
-    # light_client
-    "tests/.*/.*/light_client/single_merkle_proof",
+    # Light client sync is not implemented
     "tests/.*/.*/light_client/sync",
-    "tests/.*/electra/light_client/update_ranking",
     # LightClientStore
     "tests/.*/.*/ssz_static/LightClientStore",
     # LightClientSnapshot