server/.../backoff,syncs: don't break builds without synctest (#9484)

Previously, developers without the synctest experiment enabled would see
build failures when running tests in some server/internal/internal
packages using the synctest package. This change makes the transition to
use of the package less painful but guards the use of the synctest
package with build tags.

synctest is enabled in CI. If a new change will break a synctest
package, it will break in CI, even if it does not break locally.

The developer docs have been updated to help with any confusion about
why package tests pass locally but fail in CI.

docs/development.md

@@ -118,6 +118,35 @@ To run tests, use `go test`:
 go test ./...
 ```
 
+> NOTE: In rare cirumstances, you may nedd to change a package using the new
+> "synctest" package in go1.24.
+>
+> If you do not have the "synctest" package enabled, you will not see build or
+> test failures resulting from your change(s), if any, locally, but CI will
+> break.
+>
+> If you see failures in CI, you can either keep pushing changes to see if the
+> CI build passes, or you can enable the "synctest" package locally to see the
+> failures before pushing.
+>
+> To enable the "synctest" package for testing, run the following command:
+>
+> ```shell
+> GOEXPERIMENT=synctest go test ./...
+> ```
+>
+> If you wish to enable synctest for all go commands, you can set the
+> `GOEXPERIMENT` environment variable in your shell profile or by using:
+>
+> ```shell
+> go env -w GOEXPERIMENT=synctest
+> ```
+>
+> Which will enable the "synctest" package for all go commands without needing
+> to set it for all shell sessions.
+>
+> The synctest package is not required for production builds.
+
 ## Library detection
 
 Ollama looks for acceleration libraries in the following paths relative to the `ollama` executable:

server/internal/internal/backoff/backoff_test.go

@@ -1,3 +1,5 @@
+//go:build goexperiment.synctest
+
 package backoff
 
 import (

server/internal/internal/syncs/line_test.go

@@ -1,3 +1,5 @@
+//go:build goexperiment.synctest
+
 package syncs
 
 import (