[Security solution] assistantKnowledgeBaseByDefault flag defaults to true

[stephmilovic]

Summary

Updates the assistantKnowledgeBaseByDefault flag defaults to true, enabling the flag by default with no ability to turn it off besides reverting this PR.

This PR will be backported to 8.16, 8.x and main, but it is intended to live in 8.16 only . This PR will be followed up by a PR to remove the feature flag and all of its code paths, but that PR will only be backported to8.x and main in order to not risk many code changes in 8.16.

PR merge order

1. [Security solution] Knowledge base tour video update merged!
2. [Security Assistant] Knowledge base switch to use semantic_text) merged!
3. [Security AI Assistant] Fixed license issue for Knowledge Base resources initialization merged!
4. [Security solution] assistantKnowledgeBaseByDefault flag defaults to true (THIS PR)
5. [Security solution] assistantKnowledgeBaseByDefault flag removed

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[stephmilovic]

@elasticmachine merge upstream

[stephmilovic]

@elasticmachine merge upstream

[andrew-goldstein]

Thanks @stephmilovic!
✅ Desk tested locally
LGTM 🚀

[elasticmachine]

⏳ Build in-progress, with failures

• Buildkite Build
• Commit: a3051c7

Failed CI Steps

• FTR Configs #88
• FTR Configs #88

Test Failures

• [job] [logs] FTR Configs #88 / GenAI - Knowledge Base Entries APIs @ess Basic Security AI Assistant Knowledge Base Entries "before all" hook in "@ess Basic Security AI Assistant Knowledge Base Entries"
• [job] [logs] FTR Configs #88 / GenAI - Knowledge Base Entries APIs @ess Basic Security AI Assistant Knowledge Base Entries "before all" hook in "@ess Basic Security AI Assistant Knowledge Base Entries"