Fix auth switch writing bogus default entry into auth-contexts

[cipheraxat]

Summary

• Stop injecting a synthetic default key into auth-contexts when running doctl auth switch --context default, which previously caused writeConfig() to persist default: "true".
• Avoid mutating the caller's map in ensureDefaultContextAndKeysOrder so auth list cannot dirty Viper state before a later config write.
• Add unit and integration coverage that switching back to default leaves named contexts intact without a bogus default entry.

Fixes #1816

Test plan

• Reproduced the bug on an unfixed build: auth switch --context default wrote default: "true" under auth-contexts
• Confirmed fixed build no longer writes that entry; context: default is set and named contexts remain
• Confirmed auth list still shows default (current) and a switch roundtrip stays clean
• go test ./commands/ -run 'Auth|displayAuth|EnsureDefault'
• go test -mod=vendor ./integration -run 'TestRun/doctl/auth' (12 passed)