Fix cargo add overwriting symlinked Cargo.toml files

[RaghavenderSingh]

What does this PR try to resolve?

This PR fixes a bug where cargo add breaks symlinks to Cargo.toml files. Currently, when Cargo.toml is a symlink and cargo add is used to add a dependency, the symlink is replaced with a regular file, breaking the link to the original target file.

This issue was reported in #15241 where a user who relies on symlinked Cargo.toml files found that cargo add breaks their workflow.

Fixes #15241

How should we test and review this PR?

I've modified LocalManifest::write() to check if the path is a symlink, and if so, follow it to get the actual target path. This ensures we write to the actual file rather than replacing the symlink.

I've also added a test in tests/testsuite/cargo_add/symlink.rs that:

1. Creates a symlinked Cargo.toml file
2. Runs cargo add to add a dependency
3. Verifies the symlink is preserved and the dependency is added to the target file

I've manually tested this fix and confirmed it works correctly.

[rustbot]

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[epage]

@RaghavenderSingh you requested a review but it doesn't look like my review comments have been addressed.