[AI Assistant for Observability] Add aria-label to AI Assistant button

[CoenWarmer]

Resolves #179195

Summary

Adds an aria-label for the AI Assistant for Observability button.

[elasticmachine]

Pinging @elastic/obs-knowledge-team (Team:obs-knowledge)

[apmmachine]

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• /oblt-deploy-serverless : Deploy a serverless Kibana instance using the Observability test environments.
• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[CoenWarmer]

@elasticmachine merge upstream

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 6bf0da9

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
observabilityAIAssistantApp	140.6KB	140.8KB	+175.0B

History

• 💚 Build #200221 succeeded 1787103

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@elasticmachine merge upstream