[repo-assist] Fix trailing slash mismatch in credential/token source lookup (#3592)

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Closes #3592

Root cause

AddCredentials (ConfigFile.fs) strips the trailing slash from a source URL before storing it in paket.config:

let AddCredentials (source, username, password, authType) = 
    ...
    AddCredentials (source.TrimEnd [|'/'|], username, password, authType)

However, getSourceNodes/GetAuthenticationForUrl compared the raw lookup source (as passed at paket push time, which may or may not have a trailing slash) against the stored value with exact string equality:

let getSourceNodes (credentialsNode : XmlNode) source nodeType = 
    ...
    |> Seq.filter (fun n -> n.Attributes.["source"].Value = source)

This meant a credential/token stored for (example.com/redacted) (slash stripped on save) would never match a lookup for (example.com/redacted) (slash present, e.g. because it came from a NuGet v2 feed JSON response), causing paket push to silently skip the stored credentials and fail with 401 Unauthorized, as reported in #3592.

Fix

getSourceNodes now trims trailing / from both the lookup source and each stored source attribute before comparing, so a stored/looked-up source pair matches regardless of trailing-slash presence.

Trade-offs

• Minimal, surgical change — only affects source matching in the credential/token lookup path, no behavior change for sources that already matched exactly.
• Did not change AddCredentials's trimming behavior on save, to avoid altering existing paket.config files.

Test Status

• Build: dotnet build src/Paket.Core/Paket.Core.fsproj -f netstandard2.0 → succeeded, 0 errors.
• Added unit test get source nodes ignores trailing slash mismatch in tests/Paket.Tests/Versioning/ConfigFileSpecs.fs.
• Ran dotnet test tests/Paket.Tests/Paket.Tests.fsproj --filter "FullyQualifiedName~ConfigFileSpecs" → 3/3 passed on net9.0 (2 pre-existing tests skipped on this sandbox only because they require Windows ProtectedData.Protect for encryption; net461 host could not run due to missing mono in this sandbox — infrastructure limitation, not related to this change).

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