chore: bump Go toolchain version to 1.26.2 (#24975)

## Summary
Bumps the repository Go toolchain from 1.25.9 to 1.26.2 across local
development, CI, dogfood Docker images, and Nix builds.

## Changes
- Update `go.mod` and the shared setup-go action to Go 1.26.2.
- Update dogfood Ubuntu image Go versions and the official linux-amd64
tarball checksum.
- Move Nix Go module builds from `buildGo125Module` to
`buildGo126Module`.
- Regenerate API docs affected by Go 1.26 stdlib URL documentation
changes.

## Validation
- `./scripts/check_go_versions.sh`
- `make fmt`
- `make lint`
- `make build-slim`
- `make test TEST_SHORT=1`
- `make pre-commit`

> 🤖 This PR was created with the help of Coder Agents, and needs a human
review. 🧑💻

.github/actions/setup-go/action.yaml

@@ -4,7 +4,7 @@ description: |
 inputs:
   version:
     description: "The Go version to use."
-    default: "1.25.9"
+    default: "1.26.2"
   use-cache:
     description: "Whether to use the cache."
     default: "true"