const-oid: reject non-minimal arc encodings in BER decoder

[yuxi-liu-wired]

X.690 8.19.2 requires each subidentifier to be encoded in the fewest possible octets, so the leading octet of an arc is never 0x80. Arcs::try_next, which ObjectIdentifier::from_bytes and therefore the der decoder use, accepted that padding: 2A 80 86 48 decoded to 1.2.840, the same as the canonical 2A 86 48.

ObjectIdentifier stores and compares the original bytes, so the two encodings give OIDs that print the same but compare unequal:

let padded = ObjectIdentifier::from_bytes(&[0x2A, 0x80, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x01, 0x01, 0x01])?;
assert_eq!(padded.to_string(), "1.2.840.113549.1.1.1");
assert_ne!(padded, rsaEncryption);  // on master

DER re-encoding keeps the padding too, so a certificate with a padded OID decodes and re-encodes without error. pyca/cryptography and OpenSSL both reject the padded form.

Fix: return Error::Base128 when the first octet of an arc is 0x80, the same error that already covers a truncated arc. Arcs that only contain 0x80 after their first octet (e.g. 81 80 00) are unaffected.

from_bytes_reject_non_minimal_arc in tests/oid.rs covers padding on the first and on later arcs, on single- and multi-octet arcs, and checks that valid 0x80 octets still decode. It fails on master. cargo hack test --feature-powerset passes on stable and on 1.85 (MSRV), the thumbv7em-none-eabi powerset build succeeds, and cargo fmt --check and cargo +1.90.0 clippy --all-features --tests are clean.

Found by differential fuzzing of x509-cert against pyca/cryptography's rust-asn1. This PR was produced by AI agents (Claude), from the fuzzing and the fix through to this description.

[dishmaker]

I think one case is enough. Is it testing/covering some more lines than the 840 test?

Suggested change

	// Padding in front of a single-octet arc: `1.2.0` and `1.2.5`.
	assert_eq!(
	ObjectIdentifier::from_bytes(&[0x2A, 0x80, 0x00]),
	Err(Error::Base128),
	);
	assert_eq!(
	ObjectIdentifier::from_bytes(&[0x2A, 0x80, 0x80, 0x05]),
	Err(Error::Base128),
	);

[yuxi-liu-wired]

Done.

[dishmaker]

Suggested change

	assert_eq!(
	ObjectIdentifier::from_bytes(&[0x2A, 0x00]).unwrap().arc(2),
	Some(0),
	);

[yuxi-liu-wired]

Done.

[dishmaker]

Maybe remove this or merge with the first 840 test.

[yuxi-liu-wired]

Done.

[dishmaker]

Thanks!