Backfill contract fixtures and release conformance

[andrei-hasna]

open-mailery already depends on @hasna/contracts, but it has no .valid.json/.invalid.json fixture pair and no package script that runs contracts conformance.

Scope (this repository only):

• Keep contract parsing at the existing producer/consumer boundary (parseContract for shared contract payloads).
• Add package-owned fixtures under fixtures/ or examples/: at least one .valid.json and one .invalid.json for each adopted schema. Start from the copyable template in hasna/contracts, then replace the generic payload with this package’s real boundary payload.
• Add "contracts:conformance": "contracts conformance fixtures" (adjust the directory only if using examples/).
• Run that script and contracts no-cloud-scan . from prepublishOnly, preserving every existing typecheck, test, build, pack, and release command.

Acceptance:

• bun run contracts:conformance validates a non-empty fixture set and exits 0.
• The package prepublish gate runs conformance and no-cloud scan as well as all prior gates.
• No shared schema or runtime behavior is changed in this backfill.