test(materials): exercise trace_sessions maxLength bound

The TestValidateSecurityContextTraceSessions doc comment claimed per-item
maxLength coverage, but the rejected cases only hit maxItems, minLength, and
the array type. Add a >256-char session-id case on both commit roles so the
maxLength bound is actually exercised.

Assisted-by: Claude Code
Signed-off-by: Matías Insaurralde <matias@chainloop.dev>

Chainloop-Trace-Sessions: 3f365023-61a3-4af3-bbe9-03688f152834, 7cf89fb4-1736-425d-bafb-bfb8b59155a4, df945bde-e982-472c-bb81-50055df403bb

Author: matiasinsaurralde

internal/schemavalidators/schemavalidators_test.go

@@ -713,6 +713,7 @@ func TestValidateSecurityContextTraceSessions(t *testing.T) {
 	}{
 		{name: "more than maxItems session ids", value: overflow},
 		{name: "an empty-string session id", value: []any{""}},
+		{name: "a session id longer than maxLength", value: []any{strings.Repeat("a", 257)}},
 		{name: "a non-array trace_sessions", value: "not-an-array"},
 	}