[9.1] [Security Assistant] Prompt updates for context prompts and starter prompts (#227907)

[kibanamachine]

Backport

This will backport the following commits from main to 9.1:

• [Security Assistant] Prompt updates for context prompts and starter prompts (#227907)

Questions ?

Please refer to the Backport tool documentation

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 0b4905a

Failed CI Steps

• FTR Configs #34

Test Failures

• [job] [logs] FTR Configs #34 / Embeddable Alerts Table Embeddable alerts panel "before all" hook for "should only show alerts from the observability area (o11y+stack) when selecting it"

Metrics [docs]

✅ unchanged

cc @stephmilovic