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. 🧑💻

flake.nix

@@ -94,7 +94,7 @@
         # 3. Update the sha256 and run again
         # 4. Nix will fail with the correct vendorHash
         # 5. Update the vendorHash
-        sqlc-custom = unstablePkgs.buildGo125Module {
+        sqlc-custom = unstablePkgs.buildGo126Module {
           pname = "sqlc";
           version = "coder-fork-aab4e865a51df0c43e1839f81a9d349b41d14f05";
 
@@ -242,7 +242,7 @@
         # slim bundle into it's own derivation.
         buildFat =
           osArch:
-          unstablePkgs.buildGo125Module {
+          unstablePkgs.buildGo126Module {
             name = "coder-${osArch}";
             # Updated with ./scripts/update-flake.sh`.
             # This should be updated whenever go.mod changes!