Pin BackendID through the advertising filter (tests) (#5452)

Pin BackendID on every advertised capability

Implements tests for issue #5435 (P1.2):
- Assert BackendID != "" on every advertised Tool, Resource, and Prompt
  across the prefix, priority, and manual conflict-resolution strategies
- Cover advertising-filter modes: global ExcludeAllTools, per-workload
  ExcludeAll, and per-workload Filter
- Verify renamed/prefixed names resolve back to the backend's original
  name via GetBackendCapabilityName (the LookupTool/CallTool path)

No production change needed: BackendID is already populated at every
resolver/merge site and by the backend client.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tgrunnagle