fix(material/chips): chip label appearing above sticky columns (#26794)

Fixes that the `z-index`, which is set on the chip labels, appears on top of sticky columns inside a table.

Fixes #26793.

(cherry picked from commit e3917ea)

Author: crisbeto

src/material/chips/chip.scss

@@ -226,6 +226,11 @@
 // Required for the strong focus indicator to fill the chip.
 .mat-mdc-chip {
   position: relative;
+
+  // `.mat-mdc-chip-action-label` below sets a `z-index: 1` to put the label above the focus
+  // overlay, but that can also cause it to appear above other elements like sticky columns
+  // (see #26793). Set an explicit `z-index` to prevent the label from leaking out.
+  z-index: 0;
 }
 
 .mat-mdc-chip-action-label {