Share Viceroy fixture retargeting mechanics while preserving caller contracts #1514

Follow-up to the deferred maintenance feedback on #917: #917 (comment).

The integration harness and config-generator binary duplicate Viceroy app-config store extraction and envelope retargeting:

• crates/trusted-server-integration-tests/tests/common/config.rs::viceroy_config_with_public_origin
• crates/trusted-server-integration-tests/src/bin/generate-viceroy-config.rs::retarget_generated_config

Extract their shared mechanics without merging their distinct contracts. Preserve custom Viceroy resources and unrelated config stores, verify incoming BlobEnvelope integrity, retain the test helper's original generated_at, retain the generator's regenerated timestamp and optional upstream-origin retargeting, and preserve useful errors at both call sites.

Keep regressions for custom config resources, invalid/missing envelopes, public listener origin independent of upstream origin, and the two timestamp behaviors. This is separate from the absolute creative URL correctness fix in #917.