Resolve chained symlinks in write_atomic

[cuishuang]

What does this PR try to resolve?

Fixes #17443

write_atomic previously resolved only one symlink level. For a chain such as:

A -> B -> C

the atomic replacement was performed on B, replacing the intermediate symlink with a regular file instead of updating the final target C.

This change follows the symlink chain until it reaches the final non-symlink path. Relative targets are resolved against the parent directory of the symlink at each level, while absolute targets continue to work as before.

The implementation also detects repeated resolved paths and returns an error for symlink loops.

How to test and review this PR?

The tests cover:

• single-level relative symlinks;
• chained relative symlinks;
• updating the final target while preserving every symlink in the chain;
• symlink loops returning an error without modifying the links;
• existing absolute symlink behavior.

I ran:

cargo +1.98.0 test -p cargo-util
cargo fmt --all -- --check

[rustbot]

r? @weihanglo

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The reviewer was selected based on:

• Owners of files modified in this PR: @epage, @weihanglo
• @epage, @weihanglo expanded to epage, weihanglo
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