[SharedUX][A11y] Fix tag selector missing error text

[angeles-mb]

Closes #237856

Summary

• Added differentiated error messages for the TagSelector for the following use cases:
  • Tag doesn't exist:
    • If user has permissions, prompt to create it
    • If they don't, prompt to select one from the list
  • Tag exists, prompt to select it
  • Tag exists and it is already selected
• This change should fix all occurrences of the missing error message for the TagSelector.
• For the Settings flyout, instead of directly using the TagSelector we are now using the wrapper containing this validation logic, there should be no other changes.

Testing

Flyout Settings:

Screen.Recording.2025-10-31.at.12.03.03.mov

Dashboard Details:

Screen.Recording.2025-10-31.at.12.03.52.mov

Error announcement on VO:

[angeles-mb]

@elasticmachine run docs-build-pr

[nickpeihl]

lgtm! code review and tested roles with and without tag create capability.

[bhavyarm]

/ci

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 6260657
• Cloud Deployment

Failed CI Steps

• FTR Configs #20

Test Failures

• [job] [logs] FTR Configs #20 / discover/esql Index editor allows creation by manually adding data

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
dashboard	661.5KB	661.4KB	-129.0B

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
savedObjectsTagging	20.7KB	21.7KB	+1.0KB

History

• 💚 Build #357143 succeeded 6260657

cc @angeles-mb

[bhavyarm]

LGTM. Tested on cloud and with voiceover. So happy to see a11y fixes.