Default polling workers to CPU-based connection count (Option B: CPU-scaled)

[LukeButters]

Summary

• Polling Tentacle workers previously defaulted to 1 TCP connection, serialising all RPC calls. Under load (e.g. DRB), RPCs queue in Halibut's Pending Request Queue and time out — causing deployment failures.
• This PR implements Option B from the RFC: the default connection count scales with CPU cores, clamped to [2, 8].
  • 1-core machine → 2 connections (minimum)
  • 4-core machine → 4 connections
  • 16-core machine → 8 connections (maximum)
• The TentaclePollingConnectionCount env var still overrides the default; Halibut's MaximumActiveTcpConnectionsPerPollingSubscription cap remains in place as a server-side safeguard.
• See also: Option A PR (fixed default of 5) for the simpler alternative.

Test plan

• Verify existing tests pass
• On a 4-core worker: confirm logs show "Starting 4 polling connections"
• On a 1-core worker: confirm logs show "Starting 2 polling connections" (minimum enforced)
• Verify TentaclePollingConnectionCount=1 env var still overrides to 1 connection
• Verify TentaclePollingConnectionCount=10 is clamped to 8

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