Avoid labeling 'context' PRs as 'text-input' (flutter#157650)

flutter#157646 is being labeled by the PR labeler as "text-input" because it edited "con**text**_runner.dart". 
```
##[debug]     "**/*text*" pattern matched packages/flutter_tools/lib/src/context_runner.dart
```
https://github.com/flutter/flutter/actions/runs/11526508378/job/32090756495?pr=157646#step:2:134

Exclude the common "context" word from this label rule.

See negation example at https://github.com/actions/labeler/blob/main/README.md#basic-examples.

.github/labeler.yml

@@ -42,6 +42,8 @@
     - any-glob-to-any-file:
       - '**/text/*'
       - '**/*text*'
+    - all-globs-to-all-files:
+      - '!**/*context*'
 
 'd: api docs':
   - changed-files: