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/collab/src/llm/token.rs

@@ -2,7 +2,7 @@ use crate::db::billing_subscription::SubscriptionKind;
 use crate::db::{billing_subscription, user};
 use crate::llm::AGENT_EXTENDED_TRIAL_FEATURE_FLAG;
 use crate::{Config, db::billing_preference};
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use chrono::{NaiveDateTime, Utc};
 use jsonwebtoken::{DecodingKey, EncodingKey, Header, Validation};
 use serde::{Deserialize, Serialize};
@@ -49,7 +49,7 @@ impl LlmTokenClaims {
         let secret = config
             .llm_api_secret
             .as_ref()
-            .ok_or_else(|| anyhow!("no LLM API secret"))?;
+            .context("no LLM API secret")?;
 
         let plan = if is_staff {
             Plan::ZedPro
@@ -63,7 +63,7 @@ impl LlmTokenClaims {
         let subscription_period =
             billing_subscription::Model::current_period(Some(subscription), is_staff)
                 .map(|(start, end)| (start.naive_utc(), end.naive_utc()))
-                .ok_or_else(|| anyhow!("A plan is required to use Zed's hosted models or edit predictions. Visit https://zed.dev/account to get started."))?;
+                .context("A plan is required to use Zed's hosted models or edit predictions. Visit https://zed.dev/account to get started.")?;
 
         let now = Utc::now();
         let claims = Self {
@@ -112,7 +112,7 @@ impl LlmTokenClaims {
         let secret = config
             .llm_api_secret
             .as_ref()
-            .ok_or_else(|| anyhow!("no LLM API secret"))?;
+            .context("no LLM API secret")?;
 
         match jsonwebtoken::decode::<Self>(
             token,