Send mint-token requests with a proper Content-Type (#18334)

## Summary

Using `.body()` meant that the request was sent without a
`Content-Type`, whereas we want to explicitly communicate that the
mint-token request is a JSON payload. Using `.json()` causes request to
send the `Content-Type` header by default.

## Test Plan

Existing coverage.

---------

Signed-off-by: William Woodruff <william@astral.sh>

crates/uv-publish/Cargo.toml

@@ -36,7 +36,7 @@ futures = { workspace = true }
 glob = { workspace = true }
 itertools = { workspace = true }
 reqwest = { workspace = true }
-reqwest-middleware = { workspace = true }
+reqwest-middleware = { workspace = true, features = ["json"] }
 reqwest-retry = { workspace = true }
 rustc-hash = { workspace = true }
 serde = { workspace = true, features = ["derive"] }

crates/uv-publish/src/trusted_publishing/pypi.rs

@@ -83,7 +83,7 @@ impl TrustedPublishingService for PyPIPublishingService<'_> {
         let response = self
             .client
             .post(Url::from(mint_token_url.clone()))
-            .body(serde_json::to_vec(&mint_token_payload)?)
+            .json(&mint_token_payload)
             .send()
             .await
             .map_err(|err| {

crates/uv-publish/src/trusted_publishing/pyx.rs

@@ -109,7 +109,7 @@ impl TrustedPublishingService for PyxPublishingService<'_> {
         let response = self
             .client
             .post(Url::from(mint_token_url.clone()))
-            .body(serde_json::to_vec(&mint_token_payload)?)
+            .json(&mint_token_payload)
             .send()
             .await
             .map_err(|err| {