fix: remove invalid #[serde(flatten)] from branch deserialization test

[Copilot]

test_branch_deserialize_from_toml_rejects_empty_and_whitespace was panicking with can only flatten structs and maps instead of the expected invalid branch value error, because SerializableBranch deserializes as a plain string — incompatible with serde's flatten, which requires a struct or map.

Changes

• src/options.rs: Remove #[serde(flatten)] from the local Helper struct in the test. A plain named field is sufficient since toml::from_str with branch = "" maps directly to Helper { branch: SerializableBranch }, correctly delegating to SerializableBranch's deserializer and surfacing the invalid branch value error.
• Add #[expect(dead_code)] on the field to suppress the resulting lint (field is consumed only via deserialization).

// Before — wrong: SerializableBranch is a string, not a map
struct Helper {
    #[serde(flatten)]
    branch: SerializableBranch,
}

// After — correct
struct Helper {
    #[expect(dead_code)]
    branch: SerializableBranch,
}

[Copilot]

Pull request overview

Fixes a failing/panicking TOML deserialization test by removing an invalid #[serde(flatten)] usage on a string-backed type, ensuring the test exercises SerializableBranch’s deserializer and produces the intended “invalid branch value” error.

Changes:

• Remove #[serde(flatten)] from the test-only Helper wrapper struct used for TOML parsing.
• Add #[expect(dead_code)] to suppress the resulting “unused field” warning for a field only populated via deserialization.

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