Deduplicate the third valid_digest copy in adapters/connected.rs

[1wgrumph]

Migrated from private source issue alphazede/bran-dev#16

Follow-up from #14 (merged as 39abfdc).

#14 removed the duplicated hex/SHA-256 validator from crates/bran-core/src/agent/delegate.rs, which now uses the shared valid_sha256 in crates/bran-core/src/agent/runtime.rs.

A third byte-identical copy remains at crates/bran-core/src/adapters/connected.rs:545. It is pre-existing on main and was untouched by #14, so deduplicating it there would have been unrelated cleanup outside that change's scope.

Three implementations of the same validator must stay in sync by hand. Fold this one into the shared helper.

Low priority, maintainability only. No known correctness impact.

[1wgrumph]

Comment by @1wgrumph on original issue:

Fixed in 8682d96, merged to main (now 037e415).

The third valid_digest copy at crates/bran-core/src/adapters/connected.rs is folded into the shared valid_sha256 in crates/bran-core/src/agent/runtime.rs; one implementation now remains.

Behavior note for the record: this is not a pure no-op. The removed valid_digest used is_ascii_hexdigit() and accepted uppercase A-F; the shared valid_sha256 accepts 0-9 and a-f only. Digests are produced lowercase by ResultId::sha256, so an uppercase digest could never match a packet item anyway, and a regression test covering that case was added with the change.

Gates: 288 bran-core tests pass; ./tools/ci/check.sh --fast PASS.

[1wgrumph]

Closed in original repository (bran-dev#16).