feat: bake product version from Spec and expose build in health.

Resolve MAJOR.MINOR via EventHubSpec/VERSION; CI run_number as EVENTHUB_BUILD; health returns version/build/git_sha/built_at.

Co-authored-by: Cursor <cursoragent@cursor.com>

test/unit/admin_handler_health_tests.erl

@@ -25,6 +25,8 @@ test_health_get() ->
   ?assertEqual(<<"ok">>, maps:get(<<"status">>, Result)),
   ?assertEqual(<<"eventhub">>, maps:get(<<"service">>, Result)),
   ?assert(is_map_key(<<"version">>, Result)),
+  ?assert(is_map_key(<<"build">>, Result)),
+  ?assert(is_integer(maps:get(<<"build">>, Result))),
   ?assert(is_map_key(<<"git_sha">>, Result)).
 
 test_health_wrong_method() ->