CI failure: get-net-pwsh-versions.sh parsing .NET LTS metadata

[jmcombs]

Symptoms:

• CI run shows empty NET_RUNTIME_URL_* and multiple NET_RUNTIME_LTS_VERSION lines
• PowerShell major version sometimes parsed incorrectly

Root cause:

• .NET releases-index now has multiple active LTS channels (8.0 and 10.0). The script aggregated entries, producing multiple values and failing to resolve runtime files.
• PowerShell major version extraction cut on 'v' after stripping it, and asset selection was too broad.

Fix:

• Filter to active LTS channels and choose the highest channel-version via jq max_by(split(".") -> map(tonumber)) to select a single channel.
• Use the selected channel’s releases.json to resolve the exact runtime.files URLs.
• Compute PowerShell major via dot split and match assets by exact filename.

Impact:

• Restores correct NET_RUNTIME_URL_{x64,arm64,arm}, NET_RUNTIME_LTS_VERSION, PWSH_LTS_* variables.
• Tests (unit + integration) pass locally.

Validation:

• Unit tests against mocks pass.
• Integration tests with live endpoints pass, URLs are valid HTTPS and versions are semantic (x.y.z).