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. 🧑💻

dogfood/coder/ubuntu-22.04/Dockerfile

@@ -11,8 +11,8 @@ RUN cargo install jj-cli typos-cli watchexec-cli
 FROM ubuntu:jammy@sha256:eb29ed27b0821dca09c2e28b39135e185fc1302036427d5f4d70a41ce8fd7659 AS go
 
 # Install Go manually, so that we can control the version
-ARG GO_VERSION=1.25.9
-ARG GO_CHECKSUM="00859d7bd6defe8bf84d9db9e57b9a4467b2887c18cd93ae7460e713db774bc1"
+ARG GO_VERSION=1.26.2
+ARG GO_CHECKSUM="990e6b4bbba816dc3ee129eaeaf4b42f17c2800b88a2166c265ac1a200262282"
 
 # Boring Go is needed to build FIPS-compliant binaries.
 RUN apt-get update && \

dogfood/coder/ubuntu-26.04/Dockerfile

@@ -11,8 +11,8 @@ RUN cargo install jj-cli typos-cli watchexec-cli
 FROM ubuntu:26.04@sha256:5e275723f82c67e387ba9e3c24baa0abdcb268917f276a0561c97bef9450d0b4 AS go
 
 # Install Go manually, so that we can control the version
-ARG GO_VERSION=1.25.9
-ARG GO_CHECKSUM="00859d7bd6defe8bf84d9db9e57b9a4467b2887c18cd93ae7460e713db774bc1"
+ARG GO_VERSION=1.26.2
+ARG GO_CHECKSUM="990e6b4bbba816dc3ee129eaeaf4b42f17c2800b88a2166c265ac1a200262282"
 
 # Boring Go is needed to build FIPS-compliant binaries.
 RUN apt-get update && \