Write the closing quote when appending a synthesized license field

[devtechedge]

Fixes #315.

synthesize_manifest takes the append path when a [package] table runs to the end of the manifest, with no blank line and no following table to insert before.

That path wrote license = " followed by the expression but never wrote the closing quote, so the synthesized manifest contained license = "MIT and was not valid TOML.

This writes the closing quote, matching the sibling branch that synthesizes an entire manifest, which already terminates the value.

The expression offset is unchanged, so diagnostics still point at the start of the expression.

src/licenses/resolution.rs had no unit tests, so this adds one covering the append path.

It fails before the change with license = "MIT and passes after.

Verified locally with cargo test --features cli and cargo fmt --check.

cargo clippy --all-targets --features cli is clean apart from one pre-existing collapsible_if in is_powershell_parent, which this change does not touch.

[devtechedge]

Appreciate the merge, @Jake-Shadle.

This one was a small lesson in how the append path through synthesize_manifest differs from the whole-manifest branch, and why that missing closing quote broke the TOML.

Happy the first unit test module in resolution.rs could lock it in.

Thanks for keeping cargo-about in such good shape.