The session says whether legacy protocols are off for the account

The urn:inbuxa:jmap capability on the signed-in principal's own account
gains legacyProtocols: "enabled" or "disabled", the stricter of the
server's switch and the account's tenant's (legacy-protocols spec,
Interfaces). It is what the webmail needs to tell someone why their phone's
mail app won't connect (LP-19), and it closes acceptance test 13.

contract.md's C-1 gains the line. It is an optional field added, which
C-3 says doesn't bump the contract version.

tests/e2e/legacy_protocols.py reads it back from the session on a running
server: enabled for the tenant's user while both switches are on, disabled
once its tenant turns legacy protocols off while an account outside the
tenant still reads enabled, disabled for everyone while the server switch
is off, and enabled again at the end. All 67 checks pass.

crates/jmap/src/api/session.rs

@@ -66,9 +66,18 @@ impl SessionHandler for Server {
             Capability::Inbuxa,
             Capabilities::Empty(EmptyCapabilities::default()),
         );
+        // inbuxa: legacy-protocols, Interfaces: whichever switch is stricter
+        let legacy_protocols = if self.legacy_protocols_off_for_account(access_token).await? {
+            "disabled"
+        } else {
+            "enabled"
+        };
         account.account_capabilities.append(
             Capability::Inbuxa,
-            Capabilities::Inbuxa(InbuxaAccountCapabilities { logo }),
+            Capabilities::Inbuxa(InbuxaAccountCapabilities {
+                logo,
+                legacy_protocols,
+            }),
         );
         // inbuxa: Fastmail's Masked Email API, for accounts that may hold masks
         if access_token.has_permission(Permission::SysMaskedEmailGet) {