fix: correct TOML deserialization test for SerializableBranch empty/whitespace rejection

[Copilot]

test_branch_deserialize_from_toml_rejects_empty_and_whitespace was failing because it attempted to deserialize directly into SerializableBranch from a TOML key-value string — but TOML's top-level must be a table, so the deserializer received a map instead of a string and failed before reaching any branch validation logic.

Fix

• Wrap SerializableBranch in a helper struct for the test, matching how it actually appears in real config files:

#[derive(Debug, serde::Deserialize)]
struct Helper {
    branch: SerializableBranch,
}

let res: Result<Helper, toml::de::Error> = toml::from_str(r#"branch = """#);
// Now correctly reaches SerializableBranch::deserialize and emits "invalid branch value"

Previously, toml::from_str::<SerializableBranch>("branch = \"\"") produced invalid type: map, expected a string instead of the expected invalid branch value error.

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[Copilot]

Pull request overview

Updates a TOML deserialization test in src/options.rs to reflect how SerializableBranch is represented in real config files (as a field inside a TOML table), so the test reaches SerializableBranch’s validation logic instead of failing at the TOML top-level type.

Changes:

• Wrap SerializableBranch in a local helper struct for TOML deserialization in the test.
• Adjust the test inputs for empty and whitespace-only branch values (note: current raw strings are malformed and will still fail TOML parsing before validation).

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