chore: Even better error messages for invalid keys

[schneiderstefan]

This is a follow-up to commit 42f67fe giving fine-grained error messages for some more cases of invalid pubkeys.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 819b14b.

Security Overview

• 🔎 Scanned files: 2 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=b866ea1c-1776-41ff-a501-20d5f3214c94&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/crypto/internal/crypto_lib/basic_sig/der_utils/src/lib.rs     Improve DER parsing: add SEQUENCE/SET handling, more robust nesting depth checks, and length/empty element handling ► rs/crypto/internal/crypto_lib/basic_sig/der_utils/src/tests.rs     Add extensive tests for DER parsing, nesting depth counting, and malformed DER scenarios