Fix set AWS_REGION alongside AWS_DEFAULT_REGION

[gitfool]

Problem

PR #1336 was supposed to fix #1277 by managing $AWS_REGION alongside
$AWS_DEFAULT_REGION, but the fix was incomplete. The unset path
(eval --clear) and the guard in GetDefaultRegion were both updated to
handle $AWS_REGION, and the docs were updated to say both variables are
managed — but execShellEnvs (the function that actually sets env vars for
eval/exec/aws-sso-profile) was never updated to include AWS_REGION.

Reproduction:

aws-sso-profile dev:admin
printenv | grep AWS_REGION   # empty — only AWS_DEFAULT_REGION is set
dotnet test ...               # fails — .NET SDK only reads AWS_REGION

Changes

cmd/aws-sso/exec_cmd.go

• Extracted the region-assignment block from execShellEnvs into a new
  setRegionVars(shellVars map[string]string, region string) helper.
• setRegionVars now sets AWS_REGION alongside AWS_DEFAULT_REGION and
  AWS_SSO_DEFAULT_REGION when a region is configured.
• Updated the --no-region / -n flag help text to mention AWS_REGION.

cmd/aws-sso/exec_cmd_test.go (new file)

• Added TestSetRegionVars covering both cases:
  • non-empty region → all three vars (AWS_DEFAULT_REGION, AWS_REGION,
    AWS_SSO_DEFAULT_REGION) are set to the configured value
  • empty region → only the sentinel AWS_SSO_DEFAULT_REGION is cleared

No other changes required

Component	Status
Unset on eval --clear (eval_cmd.go)	✅ already unsets AWS_REGION
Don't override user-set region (settings.go)	✅ already reads AWS_REGION as fallback
Docs (FAQ.md, config.md)	✅ already describe both vars being managed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.52%. Comparing base (cf7edfa) to head (c6c18cd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1370   +/-   ##
=======================================
  Coverage   88.52%   88.52%           
=======================================
  Files          54       54           
  Lines        3649     3649           
=======================================
  Hits         3230     3230           
  Misses        289      289           
  Partials      130      130           

Flag	Coverage Δ
unittests	88.52% <ø> (ø)

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Thank you @gitfool !