Fix pulling metadata out of broken symlinks (#22396)

## Problem

When developing extensions locally, developers will commonly put their
source code in a specific directory. Zed uses this directory to create a
symlink starting from `$HOME/Library/Application
Support/Zed/extensions/installed` (MacOS path). When a developer then
moves this source code and tries to reinstall the extension, Zed will
fail with an unhelpful message (you can check the #Testing section).

## Change Summary

With this PR, we fix this behaviour by handling broken symlinks
specifically when returning the metadata on `fs::metadata`. Today, we

1. Pull the symlink metadata.
2. Return it if the file was not a symlink OR if it is, pull the
metadata for the pointed file.

After this change gets merged, we return the Symlink metadata if the
symlink is broken. This makes the symlink be recreated since we remove
the symlink either way.

## Risks associated with this change

It's possible changing this behaviour will show additional cases where
we are handling broken symlinks incorrectly. I expect this to be a
better scenario AND backwards compatible. We have the same behaviour we
had for 1. existing symlinks 2. normal files.

## Testing

The way I have been reproducing this is by having a private extension of
my own. I install it using the `zed: install dev extension` command
after running `RUST_LOG=debug RUST_BACKTRACE=1 scripts/zed-local -1`.
Then I move the extension to a different directory.

Zed will now keeps a broken link on its `installed` directory:

```
❯ ll
Permissions Size User    Date Modified Name
lrwxr-xr-x@    - enrikes 24 Dec 12:15  brazil-config-zed-extension -> /Volumes/workplace/BrazilConfigZedExtension
drwxr-xr-x@    - enrikes  5 Dec 14:48  java
drwxr-xr-x@    - enrikes 12 Dec 13:04  kotlin
drwxr-xr-x@    - enrikes 25 Oct 08:13  rose-pine-theme
```

Before the patch, Zed shows on its logs:

```
2024-12-24T16:44:02+01:00 INFO  extension::extension_builder] compiled Rust extension /Users/enrikes/Documents/BrazilConfigZedExtension
[2024-12-24T16:44:02+01:00 INFO  extension::extension_builder] compiling grammar brazil_config for extension /Users/enrikes/Documents/BrazilConfigZedExtension
[2024-12-24T16:44:02+01:00 INFO  extension::extension_builder] checking out brazil_config parser
[2024-12-24T16:44:04+01:00 INFO  extension::extension_builder] compiling brazil_config parser
[2024-12-24T16:44:05+01:00 INFO  extension::extension_builder] compiled grammar brazil_config for extension /Users/enrikes/Documents/BrazilConfigZedExtension
[2024-12-24T16:44:05+01:00 INFO  extension::extension_builder] finished compiling extension /Users/enrikes/Documents/BrazilConfigZedExtension
[2024-12-24T16:44:05+01:00 ERROR extensions_ui] No such file or directory (os error 2)

Stack backtrace:
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1: std::backtrace_rs::backtrace::trace_unsynchronized
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2: std::backtrace::Backtrace::create
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/backtrace.rs:331:13
   3: anyhow::error::<impl core::convert::From<E> for anyhow::Error>::from
             at /Users/enrikes/.cargo/registry/src/index.crates.io-6f17d22bba15001f/anyhow-1.0.94/src/backtrace.rs:27:14
   4: <core::result::Result<T,F> as core::ops::try_trait::FromResidual<core::result::Result<core::convert::Infallible,E>>>::from_residual
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/result.rs:1989:27
   5: <fs::RealFs as fs::Fs>::metadata::{{closure}}
             at ./crates/fs/src/fs.rs:603:13
```

After the patch, the extension is installed and the symlink replaced for
a new one pointing to the user's directory choice.

```
2024-12-24T16:53:33.916022+01:00 [INFO] compiled Rust extension /Users/enrikes/Documents/BrazilConfigZedExtension
2024-12-24T16:53:33.916094+01:00 [INFO] compiling grammar brazil_config for extension /Users/enrikes/Documents/BrazilConfigZedExtension
2024-12-24T16:53:33.916225+01:00 [INFO] checking out brazil_config parser
2024-12-24T16:53:35.481602+01:00 [INFO] compiling brazil_config parser
2024-12-24T16:53:35.964189+01:00 [INFO] compiled grammar brazil_config for extension /Users/enrikes/Documents/BrazilConfigZedExtension
2024-12-24T16:53:35.964319+01:00 [INFO] finished compiling extension /Users/enrikes/Documents/BrazilConfigZedExtension
2024-12-24T16:53:36.213608+01:00 [INFO] rebuilt extension index in 39.108542ms
2024-12-24T16:53:36.213835+01:00 [INFO] extensions updated. loading 0, reloading 1, unloading 0
2024-12-24T16:53:36.375928+01:00 [INFO] rebuilt extension index in 34.478167ms
2024-12-24T16:53:36.376054+01:00 [INFO] extensions updated. loading 0, reloading 1, unloading 0
```

and

```
❯ ll
lrwxr-xr-x@    - enrikes 24 Dec 16:53  brazil-config-zed-extension -> /Users/enrikes/Documents/BrazilConfigZedExtension
drwxr-xr-x@    - enrikes  5 Dec 14:48  java
drwxr-xr-x@    - enrikes 12 Dec 13:04  kotlin
drwxr-xr-x@    - enrikes 25 Oct 08:13  rose-pine-theme
```


Release Notes:

- Fix broken symlinks when installing dev extensions

---------

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/fs/src/fs.rs

@@ -4,7 +4,7 @@ mod mac_watcher;
 #[cfg(not(target_os = "macos"))]
 pub mod fs_watcher;
 
-use anyhow::{anyhow, Result};
+use anyhow::{anyhow, Context, Result};
 #[cfg(any(test, feature = "test-support"))]
 use git::status::FileStatus;
 use git::GitHostingProviderRegistry;
@@ -589,11 +589,19 @@ impl Fs for RealFs {
             }
         };
 
+        let path_buf = path.to_path_buf();
+        let path_exists = smol::unblock(move || {
+            path_buf
+                .try_exists()
+                .with_context(|| format!("checking existence for path {path_buf:?}"))
+        })
+        .await?;
         let is_symlink = symlink_metadata.file_type().is_symlink();
-        let metadata = if is_symlink {
-            smol::fs::metadata(path).await?
-        } else {
-            symlink_metadata
+        let metadata = match (is_symlink, path_exists) {
+            (true, true) => smol::fs::metadata(path)
+                .await
+                .with_context(|| "accessing symlink for path {path}")?,
+            _ => symlink_metadata,
         };
 
         #[cfg(unix)]