#8350 fix(chat): use hostname brand icons for inline sources

Summary

• Use existing SVG brand icons for recognized source hostnames when connector metadata is absent, including Slack workspace links.
• Preserve document and connector icon precedence, destination labels, and favicon fallbacks for unknown hosts.

Type of Change

• Bug fix

Testing

40 focused tests passed across source icons, inline links, citations and preview fetching. The hostname regression failed before the fix, and removing the suffix boundary independently fails its negative control. Browser QA covered 17 icon cases, inline previews, unchanged sizing and both themes. All nine cleanup passes, 26-workspace type-check and lint, 51 audits, generators, block-registry audit and docs-manifest check passed.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)