Use anyhow more idiomatically (#31052)

https://github.com/zed-industries/zed/issues/30972 brought up another
case where our context is not enough to track the actual source of the
issue: we get a general top-level error without inner error.

The reason for this was `.ok_or_else(|| anyhow!("failed to read HEAD
SHA"))?; ` on the top level.

The PR finally reworks the way we use anyhow to reduce such issues (or
at least make it simpler to bubble them up later in a fix).
On top of that, uses a few more anyhow methods for better readability.

* `.ok_or_else(|| anyhow!("..."))`, `map_err` and other similar error
conversion/option reporting cases are replaced with `context` and
`with_context` calls
* in addition to that, various `anyhow!("failed to do ...")` are
stripped with `.context("Doing ...")` messages instead to remove the
parasitic `failed to` text
* `anyhow::ensure!` is used instead of `if ... { return Err(...); }`
calls
* `anyhow::bail!` is used instead of `return Err(anyhow!(...));`

Release Notes:

- N/A

crates/assistant_tool/src/tool_schema.rs

@@ -27,12 +27,10 @@ fn adapt_to_json_schema_subset(json: &mut Value) -> Result<()> {
         const UNSUPPORTED_KEYS: [&str; 4] = ["if", "then", "else", "$ref"];
 
         for key in UNSUPPORTED_KEYS {
-            if obj.contains_key(key) {
-                return Err(anyhow::anyhow!(
-                    "Schema cannot be made compatible because it contains \"{}\" ",
-                    key
-                ));
-            }
+            anyhow::ensure!(
+                !obj.contains_key(key),
+                "Schema cannot be made compatible because it contains \"{key}\""
+            );
         }
 
         const KEYS_TO_REMOVE: [&str; 5] = [