Avoid repeated thread snapshot loads during subscription retries

[tim-smart]

Summary

• Track whether the HTTP thread snapshot fallback has already been attempted.
• Prevent socket subscription retries from repeatedly requesting a missing snapshot and producing recurring 404s.
• Add coverage confirming subscriptions retry while the snapshot loader runs only once.

Testing

• Added an Effect test that advances the retry clock and verifies the subscription count increases to two while the snapshot loader call count remains one.
• Not run (test results were not provided).