feat(HNT-2487): validate inbound crawl messages and the publisher list

[mmiermans]

Goal

HNT-2487

Validate the two inbound Pub/Sub job messages and the agent's publisher list at the boundary so a malformed payload fails fast rather than reaching a handler that trusts its static type. This slice adds validateCrawlArticleMessage, validateCrawlArticleDiscoveryMessage, validateLiveArticle, validatePublisherList, and the MessageValidationError they throw, each narrowing an untyped payload to its typed domain shape or throwing with the offending field named.

This builds on the domain-types slice by importing the message, corpus item, and publisher list types that these validators narrow untyped payloads to. See docs/ARCHITECTURE.md in #14 for the message contracts these validators enforce.

Implementation decisions

Decision	Approach	Why
Hand-write the validators rather than adopt a schema library	Small guards narrow an untyped payload field by field and throw an error naming the offending field	The inbound Pub/Sub messages carry only a handful of required fields, so small hand-written guards that name the offending field are simpler than adding a schema library. A schema library adds a dependency, and the compiler-plugin options add toolchain complexity, neither of which is justified at a validation surface this small. The validation library evaluation in #8 weighed Zod, Valibot, Typia, and AJV with TypeBox, leaned toward Zod if a library were adopted, and noted that the content-monorepo backend lambdas hand-write their types. Hand-writing wins here because the surface is that small, and naming the offending field keeps a poison payload easy to diagnose from the logs.
Report a validation failure as a kind distinct from a transient failure	The validators throw MessageValidationError, which the Pub/Sub validate hook reports under the validation-error kind and nacks, while at agent startup the same error aborts the process	At the subscriber boundary a malformed message must be told apart from a transient handler failure so it is not mistaken for something a redelivery would fix, matching the validate hook contract in the Pub/Sub package. At agent startup a bad publisher list should fail fast so a config error aborts rather than the agent quietly crawling nothing.
Validate shape and types without normalizing the payload	The guards reject a malformed value but return it unchanged, doing no trimming or normalization	Keeping the boundary a pure check leaves URL normalization to the single Redis key builder that hashes a URL for deduplication, so trimming lives in one place rather than being half-applied at the edge. This matches the consumer-boundary validation intent recorded for HNT-2487.
Allow a blank publisher and excerpt on a corpus item	Accept an empty string for those two fields while requiring the others non-empty	An empty excerpt is a real production state for a few genuine corpus items, so rejecting a blank excerpt would drop valid live articles. A blank publisher is different: production publishers are non-empty because the Corpus API back-fills an empty publisher with a hostname fallback on create, so allowing a blank here is defensive robustness against malformed or dev-seeded input rather than a legitimate production value. It stays harmless because publisher is an opaque pass-through the crawler echoes back to updateApprovedCorpusItem unchanged, so a blank never overwrites a real stored value.
Check status and language for string shape only, not the exact union value	Require a non-empty string and trust the producing agent for the value	Rejecting a value outside today's union would break a rolling deploy the moment a new language or status is added, against the additive-only evolution the Article Crawler tech spec calls for.
Reject a non-positive interval_minutes or refresh_interval_minutes	Guard both against zero and negatives at the boundary, beyond the type check	A non-positive interval makes every page look perpetually stale to the discovery worker, and the optional refresh window gates re-fetch in the article worker, so a zero or negative value would silently defeat deduplication.