Backport of 'Serve the Orbit MCP server without the mcp_client feature flag' to 19.4

What does this MR do and why?

Backport of !258460 (merged) to 19-4-stable-ee. Cherry-pick of fcefbcd3.

Duo agentic chat gets its MCP servers from Ai::DuoWorkflows::McpConfigService. Every server sat behind the mcp_client feature flag, including the first-party Orbit server. The flag is off by default, and GitLab Dedicated does not enable default-off flags. So on Dedicated and self-managed instances the agent has no Orbit tools, even when Orbit is enabled for the group and the user.

The fix serves the Orbit server and its pre-approved tool names without the flag. Orbit keeps its own gates in Ai::Orbit::Settings. The GitLab MCP server, the AI Catalog servers, and the x-gitlab-enabled-mcp-server-tools header stay behind the flag, so this does not start the GitLab MCP tool migration.

Why this version: the Dedicated fleet runs 19.3.x today and moves to 19.4 about five days after the 19.5 release. The 19.4 line is what Dedicated tenants receive at that upgrade, so the fix must be in 19.4.x before then.

Evidence and the end-to-end matrix are on !258460 (merged) and in gitlab-org/orbit/knowledge-graph#1036 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
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• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
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• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.