Plays the SDK's own runtime protocol (initialize, tools/list, tools/call, the bundle's mesh/ask and
mesh/state.<verb>) against mesh-mail-web's real built binary, from stdio.go and state.go's own source. The
account bucket is a real JetStream key-value bucket (mesh-mail_accounts, named as mesh-mail's own
acceptance_integration_test.go names a module's state); its fixture (accountBucketValue/accountBucketFolder)
is accounts.go's own, itself copied from mesh-mail's mirror.go AccountValue (issue 1255, 1264).
Build tag integration; compiles under go vet -tags integration ./...; never run here (go test -tags
integration is this batch's hard rule) — the lead's try runs it.
Wires 1284's per-session token into the real write form and its real POST /send; adds tests for 1284/AC1-AC4
against that real page and handler, beside this issue's own AC1-AC4.
Accounts bucket fixture (accountBucketValue/accountBucketFolder) copied from mesh-mail's own
modules/mesh-mail/cmd/mesh-mail/mirror.go AccountValue/AccountFolderValue (issue 1255); mesh-mail-web is its
own Go module, so this is a copy, not an import.
Library choice: golang.org/x/oauth2 for the code exchange, github.com/coreos/go-oidc/v3 for the ID token's
verification (issuer, audience, expiry, signature from the issuer's JWKS).
jochen