Fix placeholder cache reuse across backend switches

[alexdev03]

Summary

• include the requester backend in placeholder cache keys on proxy platforms
• prevent string and component results from one backend being reused after a server switch
• preserve the existing stable cache scope for non-proxy users
• add regression coverage for backend switches and real request timeouts

Regression evidence

• before the cache-key change, both backend-switch tests returned the cached lobby value after moving the requester to survival
• after the change, the second request is sent to the new backend and both tests pass

Verification

• ./gradlew :common:test :proxy:test --rerun-tasks --configure-on-demand --no-daemon --console=plain --max-workers=1
  • 9 tests, 0 failures/errors/skips
• ./gradlew :bukkit:compileJava :bungee:compileJava :velocity:compileJava --rerun-tasks --configure-on-demand --no-daemon --console=plain --max-workers=1
• independent review of staged diff a5225d3fd8f0b26f22394aab99caf0226086e8ba8c4cdf9e5bec09ea6a79ded9: no security or logic blockers