Fix some docs and test gaps following up multi team secret refusal

[amoghrajesh]

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Post-merge follow up to #71078, addressing review comments from
#71078 (review):

• Updated the Google Cloud Secret Manager backend docs (google-cloud-secret-manager-backend.rst)
  to reflect that the "--" ambiguous-id refusal only applies when [core] multi_team
  is enabled, replacing the outdated "rename it" advice with the actual silent
  stops-resolving-on-upgrade behavior.
• Strengthened test_ambiguous_id_resolves_when_multi_team_is_disabled in test_key_vault.py to assert the actual resolved secret name for all three lookups (connection, variable, config), not just the mocked return value.
• Aligned test_systems_manager.py with the multi_team_enabled module-level constant convention already used by the other four secrets-backend test files

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[amoghrajesh]

Unrelated failures, merging.