Add dyncfg feature flags to disable MCP endpoints without restart

[bobbyiliev]

Remove these sections if your commit already has a good description!

Motivation

Fixes https://linear.app/materializeinc/issue/SQL-114/support-feature-flag-for-disabling-mcp-server-without-environment

Description

Adding enable_mcp_agents, enable_mcp_observatory
for the MCP HTTP endpoints, checked via catalog snapshot per request.
Follows the same pattern as frontend peek sequencing per internal
discussion here.

Verification

How do you know this change is correct? Describe new or existing automated
tests, or manual steps you took.

[github-actions]

Thanks for opening this PR! Here are a few tips to help make the review process smooth for everyone.

PR title guidelines

• Use imperative mood: "Fix X" not "Fixed X" or "Fixes X"
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[ggevay]

LGTM