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/deepseek/src/deepseek.rs

@@ -29,7 +29,7 @@ impl TryFrom<String> for Role {
             "assistant" => Ok(Self::Assistant),
             "system" => Ok(Self::System),
             "tool" => Ok(Self::Tool),
-            _ => Err(anyhow!("invalid role '{value}'")),
+            _ => anyhow::bail!("invalid role '{value}'"),
         }
     }
 }
@@ -72,7 +72,7 @@ impl Model {
         match id {
             "deepseek-chat" => Ok(Self::Chat),
             "deepseek-reasoner" => Ok(Self::Reasoner),
-            _ => Err(anyhow!("invalid model id")),
+            _ => anyhow::bail!("invalid model id {id}"),
         }
     }
 
@@ -296,10 +296,10 @@ pub async fn stream_completion(
     } else {
         let mut body = String::new();
         response.body_mut().read_to_string(&mut body).await?;
-        Err(anyhow!(
+        anyhow::bail!(
             "Failed to connect to DeepSeek API: {} {}",
             response.status(),
             body,
-        ))
+        );
     }
 }