chore(tutorials): go mod tidy and add regression tests

[AdamMagued]

Problem

Running go run . in tutorials/basics or tutorials/commands (as well as go test ./... inside tutorials/) currently fails immediately with:

go: updates to go.mod needed; to update it:
	go mod tidy

Root Cause

tutorials/go.mod was pinned to Go 1.24.2 with outdated indirect dependencies that fell out of sync with the root charm.land/bubbletea/v2 module (which was bumped to Go 1.26.0 in #1807). While examples/go.mod was updated in commit fc707bb, tutorials/go.mod was not tidied.

Solution

1. Run go mod tidy in tutorials/, updating its Go directive to 1.26.0 and syncing checksums in tutorials/go.sum.
2. Add regression tests in tutorials/basics/main_test.go and tutorials/commands/main_test.go covering model initialization, updates, and views so tutorials packages are continuously verified by go test ./....

Fixes #1371

Verification

• cd tutorials && go test -v ./... (passes with both tests)
• cd tutorials/basics && go run . (runs without module error)
• cd tutorials/commands && go test -v . (passes)
• go test -race -count=1 ./... (all root package tests pass)
• cd examples && go test ./... (all examples compile and test cleanly)
• go vet ./... (passes across root, examples, and tutorials)
• git diff --check (clean, no trailing whitespace)

[AdamMagued]

Closing to keep active PRs focused on core framework issues.