Drop the unused avif feature from the image dependency (#60897)

# Objective

Every build of zed (and of every downstream gpui consumer) currently
compiles rav1e, a full AV1 video encoder, because `image`'s default
`avif` feature is enabled. That feature only provides `AvifEncoder`;
AVIF *decoding* requires the separate, non-default `avif-native` feature
(dav1d). So AVIF files do not decode anywhere in zed today, nothing
encodes AVIF (`AvifEncoder` has zero references in the repo), and rav1e
plus its support crates are dead weight in every clean build.

## Solution

Pin the workspace `image` dependency to its default feature set minus
`avif`.

No behavior changes: AVIF files fail to decode exactly as before, and
`ImageFormat::Avif` still exists (the enum is not feature-gated), so the
`image_viewer` match arm compiles unchanged. The `"avif"` entry in
gpui's supported-extension list is deliberately left untouched to keep
the diff minimal; loading such files fails identically before and after.

## Testing

- `cargo metadata` node diff (before vs after, all 1832 resolved
packages): the only change in the entire graph is `image` losing
`avif`/`default`/`default-formats`.
- `cargo tree -i rav1e` on the Linux host, plus `--target
aarch64-apple-darwin` and `--target x86_64-pc-windows-msvc`: no reverse
dependencies on any platform.
- `rg -i avif crates/` finds only string labels (an extension list in
`gpui`, a format name in `image_viewer`, an icon mapping in `theme`);
`AvifEncoder` is never referenced.
- `cargo check -p gpui` passes.
- Isolated probe crate depending on image 0.25.10 alone (cold builds,
Linux, 16 threads), default vs default-minus-avif: release drops from
191 to 122 CPU-seconds and 282 to 191 MB of `target/`; debug drops from
75 to 42 CPU-seconds and 549 to 342 MB.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments (n/a:
manifest-only change)
- [x] The content adheres to Zed's UI standards (n/a: no UI change)
- [x] Tests cover the new/changed behavior (no behavior change; CI
covers compilation)
- [x] Performance impact has been considered and is acceptable
(build-time improvement only)

---

Release Notes:

- N/A

Cargo.lock

@@ -8938,7 +8938,6 @@ dependencies = [
  "qoi",
  "ravif",
  "rayon",
- "rgb",
  "tiff",
  "zune-core",
  "zune-jpeg",

Cargo.toml

@@ -617,7 +617,25 @@ http-body = "1.0"
 httparse = "1.10"
 idna = "1.0"
 ignore = "0.4.22"
-image = "0.25.1"
+# image's default features minus "avif", which only adds an encoder (rav1e);
+# decoding AVIF would additionally need the non-default "avif-native" feature.
+image = { version = "0.25.1", default-features = false, features = [
+    "bmp",
+    "dds",
+    "exr",
+    "ff",
+    "gif",
+    "hdr",
+    "ico",
+    "jpeg",
+    "png",
+    "pnm",
+    "qoi",
+    "rayon",
+    "tga",
+    "tiff",
+    "webp",
+] }
 imara-diff = "0.1.8"
 indexmap = { version = "2.7.0", features = ["serde"] }
 indoc = "2"