Fix nested file search when starter path is a file

[modernzju]

Description

This fixes find_nested_dir_with_file so that, when the provided starter path is a file, the WalkDir traversal starts from the file's parent directory.

The function already computed starter_dir for this case, but still passed the original starter_path to WalkDir. As a result, nested files such as Forc.toml could be missed when the search started from a manifest file rather than a directory.

A targeted unit test was added to cover searching from a root Forc.toml file while skipping that root file and finding a nested Forc.toml.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[fuel-cla-bot]

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[cursor]

PR Summary

Low Risk
Single-line behavior fix in a path helper with a targeted unit test; no auth, security, or broad API changes.

Overview
find_nested_dir_with_file now passes starter_dir into WalkDir::new instead of the raw starter_path. When the starter is a file, the walk already derived the parent directory but still rooted the traversal on the file path, so nested matches (e.g. a child Forc.toml) could be skipped while the root file was ignored.

A unit test covers starting from a root manifest file, skipping that file, and finding a nested Forc.toml in a temp tree.

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