cliconfig: defer validation of relative plugin_cache_dir (#36881)

[Narita-1095305]

Overview

Running terraform -chdir=dir init with a relative
TF_PLUGIN_CACHE_DIR prints a spurious error even though the cache
works correctly. This PR implements a two-phase validation approach
to defer relative path validation until after -chdir is processed.

Reproduction Steps

1. mkdir cache test-project && cd test-project
2. echo 'terraform { required_providers { null = { source = "hashicorp/null" } } }' > main.tf
3. export TF_PLUGIN_CACHE_DIR=../cache
4. terraform -chdir=. init # works but prints false error

Fix

• Early validation: Skip stat() check for relative PluginCacheDir paths in cliconfig.Validate()
• Post-chdir validation: Added secondary validation in main.go after -chdir is processed
• Comprehensive testing:
  • Added unit tests for both absolute and relative path validation
  • Added integration test: TestPluginCacheDirWithChdir

Benefits

• ✅ Eliminates false positive errors for valid relative cache paths
• ✅ Maintains proper error detection for invalid relative paths
• ✅ Preserves existing behavior for absolute paths
• ✅ Minimal changes with maximum compatibility

Fixes #36881

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Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

None.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.

[crw]

Thanks for this submission, I will raise it in triage.

[hashicorp-cla-app]

All committers have signed the CLA.