Migrate to Tomlyn 2.x (API rename / Toml static class removed)

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Context

Dependabot PR #32 bumped Tomlyn from 0.19.0 to 2.4.1. Tomlyn 2.x
removes the top-level Toml static class and reshuffles the
parser entry points; ivi-cli's source builds against the 0.19.x
shape and would not compile without migration. Held at 0.19.0
in commit 9bada36 to keep the build green.

Affected files

• src/IviCli.Infrastructure/Mock/TomlScenarioParser.cs
• src/IviCli.Infrastructure/Configuration/TomlConfigParser.cs
• src/IviCli.Infrastructure/Auth/TomlApiTokenStore.cs
• src/IviCli.Infrastructure/Plugins/PluginLoader.cs

Acceptance

• Tomlyn pinned at the current latest stable (or whatever is
  current at follow-up time).
• All four affected files migrate to the new API surface.
• All existing tests stay green.

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Re-confirmed via Dependabot PR #42 (Tomlyn 0.19.0 → 2.5.0) on 2026-06-09. Build fails with 6 CS0103: The name 'Toml' does not exist errors across:

• src/IviCli.Infrastructure/Auth/TomlApiTokenStore.cs (read+write)
• src/IviCli.Infrastructure/Configuration/TomlConfigParser.cs
• src/IviCli.Infrastructure/Mock/TomlScenarioParser.cs
• src/IviCli.Infrastructure/Plugins/PluginLoader.cs

The legacy Toml.ToModel(string) / Toml.FromModel(object) facade was removed in Tomlyn 1.0.0 ("Remove legacy Toml facade and align new pipeline"). Migration requires rewriting these four call sites onto TomlSerializer (or TomlReader/TomlWriter). PR #42 closed; this issue remains the canonical tracker.