Bicep RPC Client: Implement pooled client factory

[anthony-c-martin]

• Introduces PooledBicepClientFactory that shares a single underlying Bicep CLI client per configuration, with lightweight wrappers that can be disposed independently
• Automatically evicts idle clients after a configurable inactivity interval (default 30s), recreating them on demand
• Extracts IJsonRpcClient interface to make BicepClient unit-testable without requiring a full CLI process
• Adds comprehensive test coverage for client pooling, eviction, JSON-RPC framing, version validation, and CLI download flows

[github-actions]

Test this change out locally with the following install scripts (Action run 27712181098)

VSCode

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-vsix.sh) --run-id 27712181098

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-vsix.ps1) } -RunId 27712181098"

Azure CLI

• Mac/Linux

  bash <(curl -Ls https://aka.ms/bicep/nightly-cli.sh) --run-id 27712181098

• Windows

  iex "& { $(irm https://aka.ms/bicep/nightly-cli.ps1) } -RunId 27712181098"

[shenglol]