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Fix: IntrinsicError when a non-directory PATH entry is used with `system_binary` (#23327)

Closes #21990

## Problem

Pants crashed with `IntrinsicError: Not a directory (os error 20)` when
a
`system_binary` target (or any binary lookup) included a non-directory
entry in
its search path. Two distinct shapes of the same underlying bug:

1. **`foo/bar` on PATH where `foo` is a file**. Calling
`path_metadata_request`
on `foo/bar` hits `ENOTDIR` in the OS. That error was not caught in the
Rust
   layer, so it surfaced as an uncaught `IntrinsicError`.

2. **A symlink-to-file on PATH** — the case from the issue report. The
first
`path_metadata_request` succeeds and returns
`PathMetadataKind::SYMLINK`,
but the Python code then tried to look up `symlink/binary_name`, which
also
   hits `ENOTDIR`.

## Fix

**`src/rust/fs/src/posixfs.rs`** — `path_metadata` now accepts a
`follow_symlinks`
flag. When `true`, it calls `tokio::fs::metadata` (which follows
symlinks) instead
of `tokio::fs::symlink_metadata`, so a symlink-to-directory appears as a
`Directory` rather than a `Symlink`. It also continues to treat
`ENOTDIR` as absent
(`Ok(None)`), fixing the file-component-on-PATH crash.

**`src/rust/fs/src/lib.rs`** — The `Vfs` trait's `path_metadata`
signature is
updated to accept `follow_symlinks: bool`. The in-memory `DigestTrie`
implementation
accepts but ignores the flag.

**`src/rust/engine/src/nodes/path_metadata.rs`** — `PathMetadataNode`
now carries
`follow_symlinks` and passes it through to the VFS call.

**`src/rust/engine/src/intrinsics/digests.rs`** — The
`path_metadata_request`
intrinsic reads `follow_symlinks` from the Python `PathMetadataRequest`
object and
passes it to `PathMetadataNode`.

**`src/python/pants/engine/fs.py`** — `PathMetadataRequest` gains a
`follow_symlinks: bool = False` field.

**`src/python/pants/core/util_rules/system_binaries.py`** — The initial
lookup of
each PATH entry uses `follow_symlinks=True`. A symlink-to-directory is
returne... (continued)

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